By user12614620 on Apr 25, 2007
One of ZFS's strong points is its ability to self-tune. It monitors the activity on the disks that it's managing, and adjusts internally for the workload being asked of it. There are a few things that ZFS cannot know, and therefore can't really tune for. These are presented as properties.
Here are a typical list of properties (as of build 62) for a filesystem or a pool. Mouseover each line for a bit more detail.
# zfs get all tankThis is the command you enter NAME PROPERTY VALUE SOURCEThe column headers tank type filesystem -The type can be a filesystem or a volume. Pools are by default filesystems tank creation Mon Apr 23 14:51 2007 -This is when the pool was created. See 'zfs history'. tank used 209K -Indicates how much of the pool is in use tank available 66.9G -Indicates how much remains within the pool tank referenced 23K -How much data is used by the pool itself, excluding filesystems and volumes within it. tank compressratio 1.00x -Current compression ratio. A value of 1.00x means compression is off or not having any effect. tank mounted yes -Is this filesystem mounted anywhere? tank quota none defaultWhat is the quota on this filesystem? tank reservation none defaultIs there any space reserved for this filesystem alone? tank recordsize 128K defaultCurrent size of the data blocks. One of the few tunables tank mountpoint /tank defaultWhere is the fielsystem mounted? tank sharenfs off defaultIs this filesystem shared via NFS? tank checksum on defaultIs block checksumming on? tank compression off defaultIs compression on? tank atime on defaultShould the access time be updated when files are read? tank devices on defaultAre device nodes allowed in this filesystem? tank exec on defaultAre executables allowed to be run from this filesystem? tank setuid on defaultIf 'off', then the setuid bit is ignored in this filesystem tank readonly off defaultReadonly or read-write filesystem? tank zoned off defaultIs this filesystem managed from a local zone? tank snapdir hidden defaultShould the .zfs directory show up in ls -a output? tank aclmode groupmask defaultControls chmod(2)'s ACL modification tank aclinherit secure defaultControls ACL inheritance tank canmount on defaultIs this filesystem mountable? tank shareiscsi off defaultShould this filesystem be shared as an iSCSI target? tank xattr on defaultAre extended attributes enabled? tank copies 1 defaultHow many copies (1-3) of data blocks should be made?
Any property in the list above whose SOURCE column is not a hypen can be modified. For example, you can change the mountpoint of the pool:
# zfs set mountpoint=/newmount tank
If the /newmount directory doesn't exist, it will be created, and the pool and its datasets will be remounted at the new location. For every property save two, the change you make to the property list has completed by the time the zfs command exits. The two properties left out of the change game are compression and copies: they only affect future writes to the filesystem.
What this means, of course, is that you can't suddenly get space from a filesystem just by turning on compression. Only new files that are written after compression is enabled will get the benefit. So we're stuck, right? Well, there's a workaround, but it's a little involved, and you need enough space to make a duplicate of the filesystem you're compressing.
Let's create a filesystem for Joe. Joe's got files that are compressible, and he'd like to use a compressed filesystem. But oops, the filesystem was created uncompressed (mouseover for more detail):
# zpool create tank c0t0d0s0Create a pool called "tank" on device c0t0d0s0 # zfs list -r tankLet see some stats NAME USED AVAIL REFER MOUNTPOINT tank 106K 66.9G 18K /tankWe're using 106K out of 67G # zfs get compression tankIs compression on? NAME PROPERTY VALUE SOURCE tank compression off defaultAnswer: nope! # zfs create tank/joefsNow let's create a filesystem for Joe to use. We'll call it joefs because we like to use our imagination
And now we email Joe and say "your filesystem is ready! It's on /tank/joefs". Joe happily creates files. We'll simulate that with mkfile:
# mkfile 2g /tank/joefs/mylargefile.dataCreate a 2 gigabyte file. mkfile's files are all zeros # zfs list -r tankNow let's see how much space we're using tank 2.00G 64.9G 19K /tankOops, we're using 2G of the pool tank/joefs 2.00G 64.9G 2.00G /tank/joefsAnd it's all in Joe's filesystem # zfs get compression tank/joefsDid we forget to turn on compression? NAME PROPERTY VALUE SOURCE tank/joefs compression off defaultYep, compression is off.
OK, so turning compression on won't change anything. Let's prove that:
# zfs set compression=on tank/joefsEnable compression # zfs get compression tank/joefsDid we do it correctly? NAME PROPERTY VALUE SOURCE tank/joefs compression on localYes, compression is on # zfs list -r tankHow much space did we save? NAME USED AVAIL REFER MOUNTPOINT tank 2.00G 64.9G 19K /tank tank/joefs 2.00G 64.9G 2.00G /tank/joefsAnswer: None at all
The workaround might not be suitable for all environments, as it involves taking down the mountpoint temporarily. The steps we need to take are to create a new filesystem with compression on, snapshot Joe's filesystem, send Joe's filesystem to the new compressed filesystem, delete Joe's filesystem, and then rename the copy to the original. Step by step:
# zfs create -o compression=on tank/compressedfsCreate a new filesystem, and this time remember to turn on compression! # zfs get -r compression tankCheck to see if it's set NAME PROPERTY VALUE SOURCE tank compression off defaultIt's still off for our pool tank/compressedfs compression on localOur new filesystem has it turned on, as expected tank/joefs compression on localThe flag hasn't changed for this filesystem - it's still on tank/joefs@snap compression - - # zfs send tank/joefs@snap | zfs receive tank/compressedfs/tmpjoeNow we get to transfer the snapshot to the compressed filesystem. We're calling it "tmpjoe" because it's just a temporary. This will take quite some time - we're copying 2 gigabytes through stdout/stdin, compressing it, and writing it to disk # zfs list -r tankNow let's see what the result of all that is NAME USED AVAIL REFER MOUNTPOINT tank 2.00G 64.9G 21K /tankStill using 2 gig tank/compressedfs 37K 64.9G 19K /tank/compressedfsOur new filesystem is only using 37k! Files that are all nulls are easy to compress tank/compressedfs/tmpjoe 18K 64.9G 18K /tank/compressedfs/tmpjoeThe temporary copy of joefs is using only 18K. Compression really works. tank/compressedfs/tmpjoe@snap 0 - 18K - tank/joefs 2.00G 64.9G 2.00G /tank/joefs tank/joefs@snap 0 - 2.00G - # ls -l /tank/joefs /tank/compressedfs/tmpjoe/Let's take a look at the two directories and see if the files are there /tank/compressedfs/tmpjoe/: total 1We're only taking up one block in the compressed filedsystem -rw------T 1 root root 2147483648 Apr 24 14:59 mylargefile.dataand yet we've got the full 2 gigabyte file here /tank/joefs: total 4194865That's a lot of blocks -rw------T 1 root root 2147483648 Apr 24 14:59 mylargefile.dataAnd it's the same size as the compressed filesystem
What's the state of compresson on our filesystems? Easy way to find out:
# zfs get -r compression tank NAME PROPERTY VALUE SOURCE tank compression off default tank/compressedfs compression on local tank/compressedfs/tmpjoe compression on inherited from tank/compressedfsA-ha. This means that tmpjoe was automatically compressed because tank/compressedfs had compression on and tmpjoe inherited that. tank/compressedfs/tmpjoe@snap compression - - tank/joefs compression off default tank/joefs@snap compression - -
Our plan now is to rename tank/compressedfs/tmpjoe to tank/joefs, but we'll still need to address the compression issue. If we just rename it, then tank/joefs will still inherit the compression=off value from tank, so any new files created in /tank/joefs will be uncompressed. Not to put too fine a point on it, but the files that are in tmpjoe will not be decompressed during the rename operation: compression (and 'copies') only affects future writes, not existing data.
So instead of inheriting the compression value, let's set it directly:
# zfs set compression=on tank/compressedfs/tmpjoe # zfs get -r compression tank NAME PROPERTY VALUE SOURCE tank compression off default tank/compressedfs compression on local tank/compressedfs/tmpjoe compression on localNo longer inherited, it has now got its own ('local') value for compression tank/compressedfs/tmpjoe@snap compression - - tank/joefs compression off default tank/joefs@snap compression - -
Now it will keep its value, no matter what dataset it is part of, and we can safely go through the steps of deleting the old filesystem and renaming the new one:
# zfs destroy -r tank/joefsThis is the nerves-inducing step, of course. If you want, you can do a 'zfs rename tank/joefs tank/saved-joefs' or something like that. Just remember to delete it once you're convinced that the rest of the steps worked # zfs rename tank/compressedfs/tmpjoe tank/joefsAnd now our old filesystem is back! And compressed! # zfs list -r tank NAME USED AVAIL REFER MOUNTPOINT tank 202K 66.9G 21K /tank tank/compressedfs 18K 66.9G 18K /tank/compressedfs tank/joefs 33K 66.9G 18K /tank/joefs tank/joefs@snap 15K - 18K - # zfs destroy tank/joefs@snapCleaning up the now-unneeded snapshot # zfs list -r tank NAME USED AVAIL REFER MOUNTPOINT tank 172K 66.9G 21K /tank tank/compressedfs 18K 66.9G 18K /tank/compressedfsNo harm in leaving this around - it may be useful and only takes up 18k of disk space tank/joefs 18K 66.9G 18K /tank/joefs # zfs get -r compression tankDouble-checking that we've got everything we wanted NAME PROPERTY VALUE SOURCE tank compression off default tank/compressedfs compression on local tank/joefs compression on localEt voilà! # ls -l /tank/joefs total 1 -rw------T 1 root root 2147483648 Apr 24 14:59 mylargefile.data # od -x /tank/joefs/mylargefile.data 0000000 0000 0000 0000 0000 0000 0000 0000 0000 \* 20000000000
Not pretty, but it's definitely doable.