Wednesday Nov 05, 2008
Wednesday Oct 01, 2008
By relling on Oct 01, 2008
I've been running Solaris NV b99 for a week or so. I've also been experimenting with the new automatic snapshot tool, which should arrive in b100 soon. To see what snapshots are taken, you can use zfs's list subcommand.
# zfs list
This is typical output and shows that my rpool (root pool) file system has a snapshot which was taken at install time: rpool@install.
But in NV b99, suddenly, the snapshots are no longer listed by default. In general, this is a good thing because there may be thousands of snapshots and the long listing is too long for humans to understand. But what if you really do want to see the snapshots? A new flag has been added to the zfs list subcommand which will show the snapshots.
# zfs list -t snapshot
This should clear things up a bit and make it easier to manage large numbers of snapshots when using the CLI. If you want to see more details on this change, see the head's up notice for PSARC 2008/469.
Tuesday Sep 02, 2008
By relling on Sep 02, 2008
We often get asked, "what is the best configuration for lots of disks" on the ZFS-discuss forum. There is no one answer to this question because you are really trading-off performance, RAS, and space. For a handful of disks, the answer is usually easy to figure out in your head. For a large number of disks, like the 48 disks found on a Sun Fire X4540 server, there are too many permutations to keep straight. If you review a number of my blogs on this subject, you will see that we can model the various aspects of these design trade-offs and compare.
A few years ago, I wrote a tool called RAIDoptimizer, which will do the math for you for all of the possible permutations. I used the output of this tool to build many of the graphs you see in my blogs.
Today, I'm making available a spreadsheet with a sample run of the permutations of a 48-disk system using reasonable modeling defaults. In this run, there are 339 possible permutations for ZFS. The models described in my previous blogs are used to calculate the values. The default values used are not representative of a specific disk, and merely represent ballpark, default values. The exact numbers are not as important as the relationships exposed for when you look at different configurations. Obviously, the tool allows us to change the disk parameters, which are usually available from disk data sheets. But this will get you into the ballpark, and is a suitable starting point for making some trade-off decisions.
For your convenience, I turned on the data filters for the columns so that you can easily filter the results. Many people also sort on the various columns. StarOffice or OpenOffice will let you manipulate the data until the cows come home. Enjoy.
Tuesday Aug 05, 2008
By relling on Aug 05, 2008
Sun has been talking more and more about enterprise-class solid-state disks (SSDs) lately. Even Jonathan blogged about it. Now we are starting to see some interesting articles hitting the press as various companies prepare to release interesting products for this market.
Today, CNET posted an interesting article by Brooke Crothers that offers some insight into how the consumer and enterprise class devices are diverging in their designs. My favorite quote is, "One of the things that SSD manufacturers have been slow to learn (is that) you can't just take a compact flash controller, throw some NAND on there and call it an SSD," said Dean Klein, vice president of memory system development at Micron. Yes, absolutely correct. Though Sun makes several products which offer compact flash (CF) for storage, the future of enterprise class SSDs is not re-badged CFs. There are many more clever tricks that can be used to provide highly reliable, fast, and reasonably priced SSDs.
Monday Jul 14, 2008
By relling on Jul 14, 2008
We have organized a ZFS Workshop for the USENIX Large Installation Systems Administration (LISA'08) conference in San Diego this November. I hope you can attend.
The call for papers describes workshops as:
One-day workshops are hands-on, participatory, interactive sessions where small groups of system administrators have an opportunity to discuss a topic of common interest. Workshops are not intended as tutorials, and participants normally have significant experience in the appropriate area, enabling discussions at a peer level. However, attendees with less experience often find workshops useful and are encouraged to discuss attendance with the workshop organizer.
There is an opportunity to seed the discussions, so be sure to let me know if there is an interesting topic to be explored.
The LISA conference is always one of the more interesting conferences for people who must deal with large sites as their day job. Many of the more difficult scalability problems are discussed in the sessions and hallways. If you are directly involved with the design or management of a large computer site, then it is an excellent conference to attend.
My first LISA was LISA-VI in 1992 where I presented a paper that Matt Long and I wrote, User-setup: A System for Custom
Configuration of User Environments, or Helping Users Help
Themselves, now hanging out on SourceForge. The original source was published on usenet -- which is how we did such things at the time. I suppose I could search around and find it archived somewhere...
Much has changed from the environments we had in 1992, but the problem of managing complex application environments continues to live on. I think that the more modern approaches to this problem, as clearly demonstrated by connected devices like the iPhone, is to leverage the internet and the browser-like interfaces to hide much of the complexity behind the scenes. In a sense, this is the approach ZFS takes to managing disks -- hide some of the mundane trivia and provide a view of storage that is more intuitive to the users of storage. The more things change, the more the problems stay the same.
Please attend LISA'08 and join the ZFS workshop.
Friday Jun 06, 2008
By relling on Jun 06, 2008
About every other month or so, someone comes onto the ZFS forum, complains about quotas, and holds up the shared /var/mail directory as an example of where UFS quotas are superior to ZFS quotas. This is becoming very irritating as it makes an assumption about /var/mail which we proved doesn't scale decades ago. Rather than trying to respond explaining this again and again, I'm blogging about it. Enjoy.
When we started building large e-mail servers using sendmail in the late 1980s, we ran right into the problem of scaling the mail delivery directory. Recall that back then relatively few people were on the internet or using e-mail, a 40MHz processor was leading edge, a 200 MByte hard disk was just becoming affordable, RAID was mostly a white paper, and e-mail attachments were very uncommon. It is often limited resources which cause scaling problems, and putting thousands of users into a single /var/mail quickly exposes issues.
Many sites implemented quotas during that era, largely because of the high cost and relative size of hard disks. The computing models were derived from the timeshare systems (eg UNIX) and that model was being stretched as network computing was evolving (qv rquotad). A common practice for Sun sites was to mount /var/mail on the NFS clients so that the mail clients didn't have to know anything about the network.
As we scaled, the first, obvious change was to centralize the /var/mail directory. This allowed you to implement a site-wide mail delivery where you could send mail to firstname.lastname@example.org instead of email@example.com. This is a cool idea and worked very well for many years. But it wasn't the best solution.
As we scaled some more, and the "administration" demanded quotas, we found that the very nature of distributed systems didn't match the quota model very well. For example, the "administration's" view was that a user may be given a quota of Q for the site. But the site now had many different file systems and a quota only really works on a single file system. We had already centralized everyone onto a single mail store and you needed some quota for the home directory and another subset of Q for the mail store. You also had to try and limit the quota on other home directories because the clever users would discover where the quotas weren't and use all of the space. Back at the mail store, it became increasingly more difficult to manage the space because, as everybody knows, the managers never delete e-mail and they complain loudly when they run out of space. So, quotas in a large, shared directory don't work very well.<\\p>
The next move was to deliver mail into the user's home directory. This is trivially easy to setup in sendmail (now). In this model, the quota only needs to be set by the userin their home directory and when they run out, you can do work. This solution bought another few years of scalability, but still has its limitations. A particularly annoying limitation is that sending mail to someone who was over quota is not handled very well. And if the sys-admins use mail to tell people they are near quota, then it might not be deliverable (recall, managers don't delete e-mail :-)
There is also a potential problem with mail bombs. In the sendmail model, each message was copied to each user's mailbox. In the old days, you could implement a policy where sendmail would reject mail messages of a large size. You can still do that today, but before attachments you could put the limit at something small, say 100 kBytes. There is no way you can do that today. So a mischievous user could send a large mail message to everyone, blow out the /var/mail directory or the quotas.
A better model is to have only one copy of an e-mail message and just use pointers for each of the recipients. But while this model can save large amounts of disk space, it is not compatible with quotas because there is no good way to assign the space to a given user.
The next problem to be solved was the clients. Using an NFS mounted /var/mail worked great for UNIX users, but didn't work very well for PCs (which were now becoming network citizens). The POP and IMAP protocols fixed this problem.
Today mail systems can scale to millions of users, but not by using a shared file system or file system quotas. In most cases, there is a database which contains info on the user and their messages. The messages themselves are placed in a database of sorts and there is usually only one copy of the message. Mail quotas can be easily implemented and the mailer can reply to a sender explaining that the recipient is over mail quota, or whatever. Automation sends a user a near-quota warning message. But this is not implemented via file system quotas.
So, please, if you want to describe shared space and file system quotas, find some other example than mail. If you can't find an example, then perhaps we can drop the whole quota argument altogether.
If your "administration" demands that you implement quotas, then you have my sympathy. Just remind them that you probably have more space in your pocket than quota on the system...
Tuesday Apr 08, 2008
By relling on Apr 08, 2008
My colleague Christine asked me some questions about my holey files posts. These are really good questions, and I'm just a little surprised that more people didn't ask them... hey, that is what the comments section is for! So, I thought I would reply publically, helping to stimulation some conversations.
Q1. How could you have a degraded pool and data corruption w/o a repair? I assume this pool must be raidz or mirror.
A1. No, this was a simple pool, not protected at the pool level. I used the ZFS copies parameter to set the number of redundant data copies to 2. For more information on how copies works, see my post with pictures.
There is another, hidden question here. How did I install Indiana such that it uses copies=2? By opening a shell and becoming root prior to beginning the install, I was able to set the copies=2 property just after the storage pool was created. By default, it gets inherited by any subsequent file system creation. Simple as that. OK, so it isn't that simple. I've also experimented with better ways to intercept the zpool create, but am not really happy with my hacks thus far. A better solution is for the installer to pick up a set of properties, but it doesn't, at least for now.
Q2. Can a striped pool be in a degraded state? Wouldn't a device faulting in that pool renders it unusable and therefore faulted?
A2. Yes, a striped storage pool can be in a degraded state. To understand this, you need to know the definitions of DEGRADED and FAULTED. Fortunately, they are right there in the zpool manual page.
One or more top-level vdevs is in the degraded state because one or more component devices are offline. Sufficient replicas exist to continue functioning.
One or more top-level vdevs is in the faulted state because one or more component devices are offline. Insufficient replicas exist to continue functioning.
By default, there are multiple replicas, so for a striped volume it is possible to be in a DEGRADED state. However, I expect that the more common case will be a FAULTED state. In other words, I do tend to recommend a more redundant storage pool: mirror, raidz, raidz2.
Q3. What does filling the corrupted part with zero do for me? It doesn't
fix it, those bits weren't zero to begin with.
A3. Filling with zeros will just make sure that the size of the "recovered" file is the same as the original. Some applications get to data in a file via a seek to an offset (random access), so this is how you would want to recover the file. For applications which process files sequentially, it might not matter.
Thursday Mar 13, 2008
By relling on Mar 13, 2008
The noerror option is described in dd(1) as:
Does not stop processing on an input error.
When an input error occurs, a diagnostic mes-
sage is written on standard error, followed
by the current input and output block counts
in the same format as used at completion. If
the sync conversion is specified, the missing
input is replaced with null bytes and pro-
cessed normally. Otherwise, the input block
will be omitted from the output.
This looks like the perfect solution, rather than my dd and iseek script. But I didn't post this because, quite simply, I don't really understand what I get out of it.
Recall that I had a corrupted file which is 2.9 MBytes in size. Somewhere around 1.1 MBytes into the file, the data is corrupted and fails the ZFS checksum test.
zpool scrub zpl_slim
Restore the file in question if possible. Otherwise restore the
Permanent errors have been detected in the following files:
I attempted to use dd with the noerror flag using several different block sizes to see what I could come up with. Here are those results:
for i in 1k 8k 16k 32k 128k 256k 512k
hmmm... all of these files are of different sizes, so I'm really unsure what I've ended up with. None of them are the same size as the original file, which is a bit unexpected.
of=/tmp/whaa.1k bs=1k conv=noerror
hmmm... well, dd did copy some of the file, but seemed to give up after around 5 attempts and I only seemed to get the first 1.1 MBytes of the file. What is going on here? A quick look at the dd source (open source is a good thing) shows that there is a definition of BADLIMIT which is how many times dd will try before giving up. The default compilation sets BADLIMIT to 5. Aha! A quick download of the dd code and I set BADLIMIT to be really huge and tried again.
of=/tmp/whbb.1k bs=1k conv=noerror
As dd processes the input file, it doesn't really do a seek, so it can't really get past the corruption. It is getting something, because od shows that the end of the whbb.1k file is not full of nulls. But I really don't believe this is the data in a form which could be useful. And I really can't explain why the new file is much larger than the original. I suspect that dd gets stuck at the corrupted area and does not seek beyond it. In any case, it appears that letting dd do the dirty work by itself will not acheive the desired results. This is, of course, yet another opportunity...
Wednesday Mar 12, 2008
By relling on Mar 12, 2008
I was RASing around with ZFS the other day, and managed to find a file which was corrupted.
zpool scrub zpl_slim
Permanent errors have been detected in the following files:
ls -ls /mnt/root/lib/amd64/libc.so.1
argv! Of course, this particular file is easily extracted from the original media, it does't contain anything unique. For those who might be concerned that it is the C runtime library, and thus very critical to running Solaris, the machine in use is only 32-bit, so the 64-bit (amd64) version of this file is never used. But suppose this were an important file for me and I wanted to recover something from it? This is a more interesting challenge...
First, let's review a little bit about how ZFS works. By default, when ZFS writes anything, it generates a checksum which is recorded someplace else, presumably safe. Actually, the checksum is recorded at least twice, just to be doubly sure it is correct. And that record is also checksummed. Back to the story, the checksum is computed on a block, not for the whole file. This is an important distinction which will come into play later. If we perform a storage pool scrub, ZFS will find the broken file and report it to you (see above), which is a good thing -- much better than simply ignoring it, like many other file systems will do.
OK, so we know that somewhere in the midst of this 2.8 MByte file, we have some corruption. But can we at least recover the bits that aren't corrupted? The answer is yes. But if you try a copy, then it bails with an error.
Since the copy was not successful, there is no destination file, not even a partial file. It turns out that cp uses mmap(2) to map the input file and copies it to the output file with a big write(2). Since the write doesn't complete correctly, it complains and removes the output file. What we need is something less clever, dd.
dd if=/mnt/root/lib/amd64/libc.so.1 of=/tmp/whee
OK, from this experiment we know that we can get about 1.2 MBytes by directly copying with dd. But this isn't all, or even half of the file. We can get a little more clever than that. To make it simpler, I wrote a little ksh script:
This script will write each of the first 23 128kByte blocks from the first argument (a file) to a unique filename as a number appended to the second argument. dd is really dumb and doesn't offer much error handling which is why I hardwired the count into the script. An enterprising soul with a little bit of C programming skill could do something more complex which handles the more general case. Ok, that was difficult to understand, and I wrote it. To demonstrate, I first appologize for the redundant verbosity:
./getaround.ksh libc.so.1 /tmp/zz
So we can clearly see that the 10th (128kByte) block is corrupted, but the rest of the blocks are ok. We can now reassemble the file with a zero-filled block.
dd if=/dev/zero of=/tmp/zz.09 bs=128k count=1
Now I have recreated the file with a zero-filled hole where the data corruption was. Just for grins, if you try to compare with the previous file, you should get what you expect.
cmp libc.so.1 /tmp/zz+
How is this useful?
Personally, I'm not sure this will be very useful for many corruption cases. As a RAS guy, I advocate many verified copies of important data placed on diverse systems and media. But most folks aren't so inclined. Everytime we talk about this on the zfs-discuss alias, somebody will say that they don't care about corruption in the middle of their mp3 files. I'm no audiophile, but I prefer my mp3s to be hole-less. So I did this little exercise to show how you can regain full access to the non-corrupted bits of a corrupted file in a more-or-less easy way. Consider this a proof of concept. There are many possible variations, such as filling with spaces instead of nulls when you are missing parts of a text file -- opportunities abound.
Wednesday Oct 03, 2007
By relling on Oct 03, 2007
Adaptec has put together a nice webinar called Nearline Data Drives and Error Handling. If you work with disks or are contemplating building your own home data server, I recommend that you take 22 minutes to review the webinar. As a systems vendor, we are often asked why we made certain design decisions to favor data over costs, and I think this webinar does a good job of showing how some of the complexity of systems design covers a large number of decision points. Here in the RAS Engineering group we tend to gravitate towards the best reliability and availability of systems, which still requires a staggering number of design trade-offs. Rest assured that we do our best to make these decisions with your data in mind.
For the ZFSers in the world, this webinar also provides some insight into how RAID systems like ZFS are designed, and why end-to-end data protection is vitally important.
Enjoy! And if you don't want your Starbuck's gift card, send it to me :-)
Tuesday Sep 18, 2007
By relling on Sep 18, 2007
Jeff Bonwick recently blogged about why ZFS uses space maps for keeping track of allocations. In my recent blog on looking at ZFS I teased you with a comment about the space map floating near the Channel Islands. Now that Jeff has explained how they work, I'll show you what they look like as viewed from space.
This is a view of a space map for a ZFS file system which was created as a recursive copy of the /usr directory followed by a recursive remove of the /usr/share directory. This allows you to see how some space is allocated and some space is free.
I wrote an add-on to NASA's Worldwind to parse zdb output looking for the space map information. Each allocation appears as a green rectangle with a starting offset and length mapped onto a square field floating above the earth. The allocations are green and the frees are yellow. The frees are also floating 100m above the allocations, though it is not easy to see from this view. Each map entry also has an optional user-facing icon which shows up as a shadowed green or yellow square. I snagged these from the StarOffice bullets images. If you hover the mouse over an icon, then a tool tip will appear showing the information about the space. In this example, the tooltip says "Free, txg=611, pass=1, offset=53fe000, size=800"
I can think of about a half dozen cool extensions to make for this, such as showing metaslab boundaries. I also need to trim the shadow field to fit; it extends too far on the right. So much to do, so little time...
Wednesday Aug 29, 2007
By relling on Aug 29, 2007
I'm walking a line - I'm thinking about I/O in motion
I'm walking a line - Just barely enough to be living
Get outta the way - No time to begin
This isn't the time - So nothing was biodone
Not talking about - Not many at all
I'm turning around - No trouble at all
You notice there's nothing around you, around you
I'm walking a line - Divide and dissolve.
[theme song for this post is Houses in Motion by the Talking Heads]
Previously, I mentioned a movie. OK, so perhaps it isn't a movie, but an animated GIF.
This is a time-lapse animation of some of the data shown in my previous blog on ZFS usage of mirrors. Here we're looking at one second intervals and the I/O to the slow disk of a two-disk mirrored ZFS file system. The workload is a recursive copy of the /usr/share directory into this file system.
The yellow areas on the device field are write I/O operations. For each time interval, the new I/O operations are shown with their latency elevators. Shorter elevators mean lower latency. Green elevators mean the latency is 10ms or less, yellow until 25ms, and red beyond 25ms. This provides some insight into the way the slab allocator works for ZFS. If you look closely, you can also see the redundant uberblock updates along the lower-right side near the edge. If you can't see that in the small GIF, click on the GIF for a larger version which is easier to see.
ZFS makes redundant copies of the metadata. By preference, these will be placed in a different slab. You can see this in the animation as there are occasionally writes further out than the bulk of the data writes. As the disk begins to fill, the gaps become filled. Interestingly, the writes to the next slab (metadata) do not have much latency - they are in the green zone. This is a simple IDE disk, so there is a seek required by these writes. This should help allay one of the fears of ZFS, that the tendency to have data spread out will be a performance problem - I see no clear evidence of that here.
I have implemented this as a series of Worldwind layers. This isn't really what Worldwind was designed to do, so there are some inefficiencies in the implementation, or it may be that there is still some trick I have yet to learn. But it is functional in that you can see I/Os in motion.
By relling on Aug 29, 2007
A few months ago, I blogged about why I wasn't at JavaOne and mentioned that I was looking at some JOGL code. Now I'm ready to show you some cool pictures which provide a view into how ZFS uses disks.
The examples here show a mirrored disk pair. I created a mirrored zpool and use the default ZFS settings. I then did a recursive copy of /usr/share into the ZFS file system. This is a write-mostly workload.
There are several problems with trying to visualize this sort of data:
- There is a huge number of data points. A 500 GByte disk has about a billion blocks. Mirror that and you are trying to visualize two billion data points. My workstation screen size is only 1.92 million pixels (1600x1200) so there is no way that I could see this much data.
- If I look at an ASCII table of this data, then it may be hundreds of pages long. Just for fun, try looking at the output of zdb -dddddd to get an idea of how the data might look in ASCII, but I'll warn you in advance, try this only on a small zpool located on a non-production system.
- One dimensional views of the data are possible. Actually, this is what zdb will show for you. There is some reasoning here because a device is accessed as a single set of blocks using an offset and size for read or write operations. But this doesn't scale well, especially to a billion data points.
- Two dimensional views are also possible, where we basically make a two dimensional array of the one dimensional data. This does hide some behaviour, as disks are two dimensional, but they are stacks of circles of different sizes. These physical details are cleverly hidden and subject to change on a per-case basis. So, perhaps we can see some info in two dimensions that would help us understand what is happening.
- Three dimensional views can show even more data. This is where JOGL comes in, it is a 3-D libary for JAVA.
It is clear that some sort of 3-D visualization system could help provide some insight into this massive amount of data. So I did it.
Where is the data going?
This is a view of the two devices in the mirror after they have been filled by the recursive copy. Yellow blocks indicate write operations, green blocks are read operations. Since this was a copy into the file system, there aren't very many reads. I would presume that your browser window is not of sufficient resolution to show the few, small reads anyway, so you'll just have to trust me.
What you should be able to see, even at a relatively low resolution, is that we're looking at a 2-D representation of each device from a 3-D viewpoint. Zooming, panning, and moving the viewpoint allows me to observe more or less detail.
To gather this data, I used TNF tracing. I could also write a dtrace script to do the same thing. But I decided to use TNF data because it has been available since Solaris 8 (7-8 years or so) and I have an archive of old TNF traces that I might want to take a look at some day. So what you see here are the I/O operations for each disk during the experiment.
How long did it take? (Or, what is the latency?)
The TNF data also contains latency information. The latency is measured as the difference in time between the start of the I/O and its completion. Using the 3rd dimension, I put the latency in the Z-axis.
Ahhh... this view tells me something interesting. The latency is shown as a line emitting from the starting offset of the block being written. You can see some regularity over the space as ZFS will coalesce writes into 128 kByte I/Os. The pattern is more clearly visible on the device on the right.
But wait! What about all of the red? I color the latency line green when the latency is less than 10ms, yellow until 25ms, and red for latency > 25ms. The height of the line is a multiple of its actual latency. Wow! The device on the left has a lot of red, it sure looks slow. And it is. On the other hand, the device on the right sure looks fast. And it is. But this view is still hard to see, even when you can fly around and look at it from different angles. So, I added some icons...
I put icons at the top of the line. If I hover the mouse over an icon, it will show a tooltip which contains more information about that data point. In this case, the tooltip says, "Write, block=202688, size=64, flags=3080101, time=87.85" The size is in blocks, the flags are defined in a header file somewhere, and the time is latency in milliseconds. So we wrote 32 kBytes at block 202,688 in 87.85 ms. This is becoming useful! By cruising around, it becomes apparent that for this slow device, small writes are faster than large writes, which is pretty much what you would expect.
Finding a place in the world
Now for the kicker. I implemented this as an add-on to NASA's Worldwind.
I floated my devices at 10,000 m above the ocean off the west coast of San Diego! By leveraging the Worldwind for Java SDK, I was able to implement my visualization by writing approximately 2,000 lines of code. This is a pretty efficient way of extending a GIS tool into non-GIS use, while leveraging the fact that GIS tools are inherently designed to look at billions of data points in 3-D.
More details of the experiment
The two devices are intentionally very different from a performance perspective. The device on the left is an old, slow, relatively small IDE disk. The device on the right is a ramdisk.
I believe that this technique can lead to a better view of how systems work under the covers, even beyond disk devices. I've got some cool ideas, but not enough days in the hour to explore them all. Drop me a line if you've got a cool idea.
The astute observer will notice another view of the data just to the north of the devices. This is the ZFS space map allocation of one of the mirror vdevs. More on that later... I've got a movie to put together...
Friday May 04, 2007
By relling on May 04, 2007
OpenSolaris build 61 (or later) is now available for download. ZFS has added a new feature that will improve data protection: redundant copies for data (aka ditto blocks for data). Previously, ZFS stored redundant copies of metadata. Now this feature is available for data, too.
This represents a new feature which is unique to ZFS: you can set the data protection policy on a per-file system basis, beyond that offered by the underlying device or volume. For single-device systems, like my laptop with its single disk drive, this is very powerful. I can have a different data protection policy for the files that I really care about (my personal files) than the files that I really don't care about or that can be easily reloaded from the OS installation DVD. For systems with multiple disks assembled in a RAID configuration, the data protection is not quite so obvious. Let's explore this feature, look under the hood, and then analyze some possible configurations.
To change the numbers of data copies, set the copies property. For example, suppose I have a zpool named "zwimming." The default number of data copies is 1. But you can change that to 2 quite easily.
# zfs set copies=2 zwimming
The copies property works for all new writes, so I recommend that you set that policy when you create the file system or immediately after you create a zpool.
You can verify the copies setting by looking at the properties.
# zfs get copies zwimming NAME PROPERTY VALUE SOURCE zwimming copies 2 local
ZFS will account for the space used. For example, suppose I create three new file systems and copy some data to them. You can then see that the space used reflects the number of copies. If you use quotas, then the copies will be charged against the quotas, too.
# zfs create -o copies=1 zwimming/single # zfs create -o copies=2 zwimming/dual # zfs create -o copies=3 zwimming/triple # cp -rp /usr/share/man1 /zwimming/single # cp -rp /usr/share/man1 /zwimming/dual # cp -rp /usr/share/man1 /zwimming/triple # zfs list -r zwimming
This makes sense. Each file system has one, two, or three copies of the data and will use correspondingly one, two, or three times as much space to store the data.
Under the Covers
ZFS will spread the ditto blocks across the vdev or vdevs to provide spatial diversity. Bill Moore has previously blogged about this, or you can see it in the code for yourself. From a RAS perspective, this is a good thing. We want to reduce the possibility that a single failure, such as a drive head impact with media, could disturb both copies of our data. If we have multiple disks, ZFS will try to spread the copies across multiple disks. This is different than mirroring, in subtle ways. The actual placement is ultimately based upon available space. Let's look at some simplified examples. First, for the default file system configuration settings on a single disk.
Note that there are two copies of the metadata, by default. If we have two or more copies of the data, the number of metadata copies is three.
Suppose you have a 2-disk stripe. In that case, ZFS will try to spread the copies across the disks.
Since the copies are created above the zpool, a mirrored zpool
will faithfully mirror the copies.
Since the copies policy is set at the file system level, not the zpool level, a single zpool may contain multiple file systems, each with different policies. In other words, you could have data which is not copied allocated along with data that is copied.
Using different policies for different file systems allows you to have different data protection policies, allows you to improve data protection, and offers many more permutations of configurations for you to weigh in your designs.
It is obvious that increasing the number of data copies will effectively reduce the amount of available space accordingly. But how will this affect reliability? To answer that question we use the MTTDL model I previously described, with the following changes:
First, we calculate the probability of unsuccessful reconstruction due to a UER for N disks of a given size (unit conversion omitted). The number of copies decreases this probability. This makes sense as we could use another copy of the data for reconstruction and to completely fail, we'd need to lose all copies:
Precon_fail = ((N-1) \* size / UER)copies
For single-disk failure protection:
MTTDL = MTBF / (N \* Precon_fail)
For double-disk failure protection:
MTTDL = MTBF2/ (N \* (N-1) \* MTTR \* Precon_fail)
Note that as the number of copies increases, Precon_fail approaches zero quickly. This will increase the MTTDL. We want higher MTTDL, so this is a good thing.
OK, now that we can calculate available space and MTTDL, let's look at some configurations for 46 disks available on a Sun Fire X4500 (aka Thumper). We'll look at single parity schemes, to reduce the clutter, but double parity schemes will show the same, relative improvements.
You can see that we are trading off space for MTTDL. You can also see that for raidz zpools, having more disks in the sets reduces the MTTDL. It gets more interesting to see that the 2-way mirror with copies=2 is very similar in space and MTTDL to the 5-disk raidz with copies=3. Hmm. Also, the 2-way mirror with copies=1 is similar in MTTDL to the 7-disk raidz with copies=2, though the mirror configurations allow more space. This information may be useful as you make trade-offs. Since the copies parameter is set per file system, you can still set the data protection policy for important data separately from unimportant data. This might be a good idea for some situations where you might have permanent originals (eg. CDs, DVDs) and want to apply a different data protection policy.
In the future, once we have a better feel for the real performance considerations, we'll be able to add a performance component into the analysis.
Single Device Revisited
Now that we see how data protection is improved, let's revisit the single device case. I use the term device here because there is a significant change occurring in storage as we replace disk drives with solid state, non-volatile memory devices (eg. flash disks and future MRAM or PRAM devices). A large number of enterprise customers demand dual disk drives for mirroring root file systems in servers. However, there is also a growing demand for solid state boot devices, and we have some Sun servers with this option. Some believe that by 2009, the majority of laptops will also have solid state devices instead of disk drives. In the interim, there are also hybrid disk drives.
What affect will these devices have on data retention? We know that if the entire device completely fails, then the data is most likely unrecoverable. In real life, these devices can suffer many failures which result in data loss, but which are not complete device failures. For disks, we see the most common failure is an unrecoverable read where data is lost from one or more sector (bar 1 in the graph below). For flash memories, there is an endurance issue where repeated writes to a cell may reduce the probability of reading the data correctly. If you only have one copy of the data, then the data is lost, never to be read correctly again.
We captured disk error codes returned from a number of disk drives in the field. The Pareto chart below shows the relationship between the error codes. Bar 1 is the unrecoverable read which accounts for about 24% of the errors recorded. The violet bars show recoverable errors which did succeed. Examples of successfully recovered errors are: write error - recovered with block reallocation, read error - recovered by ECC using normal retries, etc. The recovered errors do not (immediately) indicate a data loss event, so they are largely transparent to applications. We worry more about the unrecoverable errors.
Approximately 1/3 of the errors were unrecoverable. If such an error occurs in ZFS metadata, then ZFS will try to read alternate metadata copy and repair the metadata. If the data has multiple copies, then it is likely that we will not lose any data. This is a more detailed view of the storage device because we are not treating all failures as a full device failure.
Both real and anecdotal evidence suggests that unrecoverable errors can occur while the device is still largely operational. ZFS has the ability to survive such errors without data loss. Very cool. Murphy's Law will ultimately catch up with you, though. In the case where ZFS cannot recover the data, ZFS will tell you which file is corrupted. You can then decide whether or not you should recover it from backups or source media.
Another Single Device
Now that I've got you to think of the single device as a single device, I'd like to extend the thought to RAID arrays. There is much confusion amongst people about whether ZFS should or should not be used with RAID arrays. If you search, you'll find comments and recommendations both for and against using hardware RAID for ZFS. The main argument is centered around the ability of ZFS to correct errors. If you have a single device backed by a RAID array with some sort of data protection, then previous versions of ZFS could not recover data which was lost. Hold it right there, fella! Do I mean that RAID arrays and the channel from the array to main memory can have errors? Yes, of course! We have seen cases where errors were introduced somewhere along the path between disk media to main memory where data was lost or corrupted. Prior to ZFS, these were silent errors and blissfully ignored. With ZFS, the checksum now detects these errors and tries to recover. If you don't believe me, then watch the ZFS forum on opensolaris.org where we get reports like this about once a month or so. With ZFS copies, you can now recover from such errors without changing the RAID array configuration.
If ZFS can correct a data error, it will attempt to do so. You now have a the option to improve your data protection even when using a single RAID LUN. And this is the same mechanism we can use for a single disk or flash drive: data copies. You can implement the copies on a per-file system basis and thus have different data protection policies even though the data is physically stored on a RAID LUN in a hardware RAID array. I really hope we can put to rest the "ZFS prefers JBOD" argument and just concentrate our efforts on implementing the best data protection policies for the requirements.
ZFS with data copies is another tool in your toolbelt to improve your life, and the life of your data.
Wednesday Apr 18, 2007
By relling on Apr 18, 2007
eWeek has published a nice article describing Sun's new, low-cost RAID array: the Sun StorageTek ST2500 Low Cost Array. This is an interesting new product that has broad appeal and will be a heck of a good box to run under ZFS.
But I'm worried about eWeek. It seems that they've lost track of time. Many of us have been running on internet time for most of our lives. This quote makes me wonder if eWeek forgot to update their timezone data:
"The ZFS [the speedy Zeta[sic] file system, recently released to the open-source community by Sun] is very interesting, and people are looking at it," [Henry] Baltazar told eWeek.
I'm pretty sure Henry Baltazar is running on internet time, and provided a very nice quote. But whoever added the editorial clarification at eWeek spelled Zettabyte wrong and is running on island time. ZFS was released to the open-source community on June 14, 2005 - nearly 2 years ago in real time. Even in real time, 2 years can hardly be considered "recent." Sigh.
- Gotta love the spring
- Roch star and Really Cool Stuff
- Oldie but goodie
- Evolution of RAS in the Sun SPARC T5440 server
- Hey! Where did my snapshots go? Ahh, new feature...
- Sample RAIDoptimizer output
- Dependability Benchmarking for Computer Systems
- Smartphones will rule the earth!
- More Enterprise-class SSDs Coming Soon
- ZFS Workshop at LISA'08