ZFS: filesystem properties

There is quite a number of ZFS filesystems properties, and I will cover most useful ones today.

I’ll begin with refreshing your knowledge a bit. As I’ve told before, I’m going to create a number of ZFS filesystems on my server for the next few blog entires on ZFS. So, right now I have the following:

solaris# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
stock                 2.40G  31.1G  8.50K  /stock
stock/oracle          2.32G   693M  2.32G  /stock/oracle
stock/try                8K   512M     8K  /try

To get a value of some ZFS property, we normally use a command like this:

solaris# zfs get used stock/try

NAME             PROPERTY       VALUE                      SOURCE
stock/try        used           8K                         -

But such output isn’t always convenient for us – it’s formatted this way to make the data more human readable. But if we’re going to use

solaris# zfs get

to obtain some ZFS parameters from our scripts, we really don’t need anything fancy. So we’re probably better off using a special -H command line option:

solaris# zfs get -H used stock/try
stock/try       used    8K      -

One more thing – a useful, yet very obvious feature which allows you specifying few properties names in one command line:

solaris# zfs get -H used,available,mounted stock/try
stock/try       used    8K      -
stock/try       available       512M    -
stock/try       mounted yes     -

Okay, now let’s move on and look at some useful ZFS filesystems properties.

For instance, the mountpoint property name speaks for itself. By default, the filesystem which you change the mountpoint for, will be automatically remounted at a new location:

solaris# zfs set mountpoint=/younameit stock/try
solaris# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
stock                 2.40G  31.1G  8.50K  /stock
stock/oracle          2.32G   693M  2.32G  /stock/oracle
stock/try                8K   512M     8K  /younameit
solaris# df -k | grep stock
stock                35112960       8 32597477     1%    /stock
stock/oracle         3145728 2436188  709540    78%    /stock/oracle
stock/try             524288       8  524280     1%    /younameit

One more potentially useful option is an on-disk compression. It happens on the fly, and it is planned to eventually have a number of compression methods available to a ZFS administrator, but at the moment you can use only one method – lzjb.

Still, results are impressive:

solaris# ls -l /younameit/access_log
total 3082
drwxr-xr-x   2 root     sys            3 Jan 23 21:59 .
drwxr-xr-x  44 root     root        1024 Jan 23 21:45 ..
dr-xr-xr-x   3 root     root           3 Jan 23 22:04 .zfs
-rw-r--r--   1 root     root     8904090 Jan 23 21:59 access_log
solaris# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
stock                 2.41G  31.1G  8.51M  /stock
stock/oracle          2.32G   693M  2.32G  /stock/oracle
stock/try             1.51M   510M  1.51M  /younameit

No wonder the compression ratio is so good for this filesystem:

solaris# zfs get compressratio stock/try
NAME             PROPERTY       VALUE                      SOURCE
stock/try        compressratio  5.63x                      -

Unfortunately, only few types of files can be so wonderfully compressed, and text files (for instance, my Apache2 logs) belong to them too. Executable files, of course, will compress with a much lower ratio.

I think that should do for today’s entry. Let me tell you right now: I have a whole series of ZFS blog entries planned ahead, and that’s the reason I haven’t covered some of the simpler ZFS filesystem options yet. I promise to write more in the nearest few days.