Thursday Sep 23, 2010

Leaving Oracle

I first came to Sun over 4 years ago for an internship in the Solaris kernel group.  I was excited to work with such a highly regarded group of talented engineers, and my experience that summer was no disappointment: I learned a lot and had a blast.

After college, I joined Sun's Fishworks team.  Despite my previous experience at Sun, I didn't really know what to expect here, and I certainly didn't imagine then where I'd be now and how much I'd value the experiences of the last three years.  In that short time, I've worked with many bright people with diverse backgrounds, I've learned so much about so many topics (both technical and otherwise), and I've continued having fun contributing to a great product.

Now I've decided to try something different, this time outside Sun/Oracle.  My last day here is 9/24.  While I'm excited about the future, I'll miss both the people and the work here.  I wish you all the best of luck, and thanks for everything!

Update: This blog has moved to dtrace.org, where comments are open for this entry.

Wednesday Sep 22, 2010

SS7000 Software Updates

In this entry I'll explain some of the underlying principles around software upgrade for the 7000 series.  Keep in mind that nearly all of this information are implementation details of the system and thus subject to change.

Entire system image

One of the fundamental design principles about SS7000 software updates is that all releases update the entire system no matter how small the underlying software change is. Releases never update individual components of the system separately. This sounds surprising to people familiar with traditional OS patches, but this model provides a critical guarantee: for a given release, the software components are identical across all systems running that release. This guarantee makes it possible to test every combination of software the system can run, which isn't the case for traditional OS patches.

When operating systems allow users to apply separate patches to fix individual issues, different systems running the same release (obviously) may have different patches applied to different components. It's impossible to test every combination of components before releasing each new patch, so engineers rely heavily on understanding the scope of the underlying software change (as it affects the rest of the system at several different versions) to know which combinations of patches may conflict with one other. In a complex system, this is very hard to get right. What's worse is that it introduces unnecessary risk to the upgrade and patching process, making customers wary of upgrading, which results in more customers running older software. With a single system image, we can (and do) test every combination of component versions a customer can have.

This model has a few other consequences, some of which are more obvious than others:

  • Updates are complete and self-contained. There's no chance for interaction between older and newer software, and there's no chance for user error in applying illegal combinations of patches.
  • An update's version number implicitly identifies the versions of all software components on the system. This is very helpful for customers, support, and engineering to exactly what it means to say a bug was fixed in release X or that a system is running release Y (without having to guess or specify the versions of dozens of smaller components).
  • Updates are cumulative; any version of the software has all bug fixes from all previous versions. This makes it easy to identify which releases have a given bug fixed. "Previous" here refers to the version order, not chronological order. For example, V1 and V3 may be in the field when a V2 is released that contains all fixes in V1 plus a few others, but not the fixes in V3. More below on when this happens.
  • Updates are effectively atomic. They either succeed or they fail, but the system is never left in some in-between state running partly old bits and partly new bits. This doesn't follow immediately from having an entire system image, but doing it that way makes this possible.
Of course, it's much easier to achieve this model in the appliance context (which constrains supported configurations and actions for exactly this kind of purpose) than on a general purpose operating system.

Types of updates

SS7000 software releases come in a few types characterized by the scope of the software changes contained in the update.

  • Major updates are the primary release vehicle. These are scheduled releases that deliver new features and the vast majority of bug fixes. Major updates generally include a complete sync with the underlying Solaris OS.
  • Minor updates are much smaller, scheduled releases that include a small number of valuable bug fixes. Bugs fixed in minor updates must have high impact or high likelihood of being experienced by customers and have a relatively low risk fix.
  • Micro updates are unscheduled releases issued to address significant issues. These would usually be potential data loss issues, pathological service interruptions (e.g., frequent reboots), or significant performance regressions.

Enterprise customers are often reluctant to apply patches and upgrades to working systems, since any software change carries some risk that it may introduce new problems (even if it only contains bug fixes). This breakdown allows customers to make risk management decisions about upgrading their systems based on the risk associated with each type of update. In particular, the scope of minor and micro releases is highly constrained to minimize risk.

For examples, four major software updates have been released: 2008.Q4, 2009.Q2, and 2009.Q3, and 2010.Q1. The first four of these have had several updates. We've also released a few micro updates.

Tuesday Sep 21, 2010

Replication for disaster recovery

When designing a disaster recovery solution using remote replication, two important parameters are the recovery time objective (RTO) and the recovery point objective (RPO). For these purposes, a disaster is any event resulting in permanent data loss at the primary site which requires restoring service using data recovered from the disaster recovery (DR) site. The RTO is how soon service must be restored after a disaster. Designing a DR solution to meet a specified RTO is complex but essentially boils down to ensuring that the recovery plan can be executed within the allotted time. Usually this means keeping the plan simple, automating as much of it as possible, documenting it carefully, and testing it frequently. (It helps to use storage systems with built-in features for this.)

In this entry I want to talk about RPO.  RPO describes how recent the data recovered after the disaster must be (in other words, how much data loss is acceptable in the event of disaster). An RPO of 30 minutes means that the recovered data must include all changes up to 30 minutes before the disaster.  But how do businesses decide how much data loss is acceptable in the event of a disaster? The answer varies greatly from case to case. Photo storage for a small social networking site may have an RPO of a few hours; in the worst case, users who uploaded photos a few hours before the event will have to upload them again, which isn't usually a big deal. A stock exchange, by contrast, may have a requirement for zero data loss: the disaster recovery site absolutely must have the most updated data because otherwise different systems may disagree about whose account owns a particular million dollars. Of course, most businesses are somewhere in between: it's important that data be "pretty fresh," but some loss is acceptable.

Replication solutions used for disaster recovery typically fall into two buckets: synchronous and asynchronous. Synchronous replication means that clients making changes on disk at the primary site can't proceed until that data also resides on disk at the disaster recovery site. For example, a database at the primary site won't consider a transaction having completed until the underlying storage system indicates that the changed data has been stored on disk. If the storage system is configured for synchronous replication, that also means that the change is on disk at the DR site, too. If a disaster occurs at the primary site, there's no data loss when the database is recovered from the DR site because no transactions were committed that weren't also propagated to the DR site. So using synchronous replication it's possible to implement a DR strategy with zero data loss in the event of disaster -- at great cost, discussed below.

By contrast, asynchronous replication means that the storage system can acknowledge changes before they've been replicated to the DR site. In the database example, the database (still using synchronous i/o at the primary site) considers the transaction completed as long as the data is on disk at the primary site. If a disaster occurs at the primary site, the data that hasn't been replicated will be lost. This sounds bad, but if your RPO is 30 minutes and the primary site replicates to the target every 10 minutes, for example, then you can still meet your DR requirements.

Synchronous replication

As I suggested above, synchronous replication comes at great cost, particularly in three areas:

  • availability: In order to truly guarantee no data loss, a synchronous replication system must not acknowledge writes to clients if it can't also replicate that data to the DR site. If the DR site is unavailable, the system must either explicitly fail or just block writes until the problem is resolved, depending on what the application expects. Either way, the entire system now fails if any combination of the primary site, the DR site, or the network link between them fails, instead of just the primary site. If you've got 99% reliability in each of these components, the system's probability of failure goes from 1% to almost 3% (1 - .993).
  • performance: In a system using synchronous replication, the latency of client writes includes the time to write to both storage systems plus the network latency between the sites. DR sites are typically located several miles or more from the primary site in case the disaster takes the form of a datacenter-wide power outage or an actual natural disaster. All things being equal, the farther the sites are apart, the higher the network latency between them. To make things concrete, consider a single threaded client that sees 300us write operations at the primary site (total time including network round trip plus time to write the data to local storage only, not the remote site). Add a synchronous replication target a few miles away with just 500us latency from the primary site and the operation now takes 800us, dropping IOPS from about 3330 to about 1250.
  • money: Of course, this is what it ultimately boils down to. Synchronous replication software alone can cost quite a bit, but you can also end up spending a lot to deal with the above availability and performance problems: clustered head nodes at both sites, redundant network hardware, and very low latency switches and network connection. You might buy some of this for asynchronous replication, too, but network latency is much less important than bandwidth for asynchronous replication so you often can  save on the network side.

Asynchronous replication

The above availability costs scale back as the RPO increases.  If the RPO is 24 hours and it only takes 1 hour to replicate a day's changes, then you can sustain a 23-hour outage at the DR site or on the network link without impacting primary site availability at all.  The only performance cost of asynchronous replication is the added latency resulting from a system's additional load for the replication software (which you'd also have to worry about with synchronous replication).  There's no additional latency from the DR network link or the DR site as long as the system is able to keep up with sending updates.

The 7000 series provides two types of automatic asynchronous replication: scheduled and continuous. Scheduled replication sends discrete, snapshot-based updates at predefined hourly, daily, weekly, or monthly intervals. Continuous replication sends the same discrete, snapshot-based updates as frequently as possible.  The result is essentially a continuous stream of filesystem changes to the DR site.

Of course, there are tradeoffs to both of these approaches. In most cases, continuous replication minimizes data loss in the event of a disaster (i.e., it will achieve minimal RPO), since the system is replicating changes as fast as possible to the DR site without actually holding up production clients. However, if the RPO is a given parameter (as it often is), you can just as well choose a scheduled replication interval that will achieve that RPO, in which case using continuous replication doesn't buy you anything.  In fact, it can hurt because continuous replication can result in transferring significantly more data than necessary. For example, if an application fills a 10GB scratch file and rewrites it over the next half hour, continuous replication will send 20GB, while half-hourly scheduled replication will only send 10GB.  If you're paying for bandwidth, or if the capacity of a pair of systems is limited by the available replication bandwidth, these costs can add up.

Conclusions

Storage systems provide many options for configuring remote replication for disaster recovery, including synchronous (zero data loss) and both continuous and scheduled asynchronous replication.  It's easy to see these options and guess a reasonable strategy for minimizing data loss in the event of disaster, but it's important that actual recovery point objectives be defined based on business needs and that those objectives drive the planning and deployment of the DR solution. Incidentally, some systems use a hybrid approach that uses synchronous replication when possible, but avoids the availability cost (described above) of that approach by falling back to continuous asynchronous replication if the DR link or or DR system fails. This seems at first glance a good compromise, but it's unclear what problem this solves because the resulting system pays the performance and monetary costs of synchronous replication without actually guaranteeing zero data loss.

About

On Fishworks, Sun, and software engineering

Search

Categories
Archives
« September 2010
SunMonTueWedThuFriSat
   
1
2
3
4
5
6
7
8
10
11
12
13
14
15
16
17
18
19
20
24
25
26
27
28
29
30
  
       
Today