solaris unix zfs file system II

Getting Started

You need:

• An operating system with ZFS support:
  • Solaris 10 6/06 or later 
  • OpenSolaris (discontinued)
  • FreeBSD 7 or later
  • Mac OS X with maczfs 
  • Linux with ZFS kernel module
• Root privileges (or a role with the appropriate ZFS rights profile)
• Some storage, either:
  • 256 MB of disk space on an existing partition
  • Two spare disks of the same size

Using Files

To use files on an existing filesystem, create two 128 MB files, eg.:

# mkfile 128m /home/ocean/disk1
# mkfile 128m /home/ocean/disk2

# ls -lh /home/ocean
total 1049152
-rw------T   1 root     root        128M Mar  7 19:48 disk1
-rw------T   1 root     root        128M Mar  7 19:48 disk2

Using Disks

To use real disks in the tutorial make a note of their names (eg. c2t1d0 or c1d0 under Solaris). You will be destroying all the partition information and data on these disks, so be sure they’re not needed.

In the examples I will be using a pair of 146 GB disks named c3t2d0 and c3t3d0; substitute your disks or files for them as appropriate.

ZFS Filesystems

ZFS filesystems within a pool are managed with the zfs command. Before you can manipulate filesystems you need to create a pool (you can learn about ZFS pools in part 1). When you create a pool, a ZFS filesystem is created and mounted for you.

ZFS Filesystem Basics

Create a simple mirrored pool and list filesystem information with zfs list:

# zpool create salmon mirror c3t2d0 c3t3d0

# zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
salmon                  136G   84.5K    136G     0%  ONLINE     -

# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
salmon                75.5K   134G  24.5K  /salmon

We can see our filesystem is mounted on /salmon and is 134 GB in size.

We can create an arbitrary number (264) of new filesystems within our pool. Let’s add some filesystems space for three users with zfs create:

# zfs create salmon/kent
# zfs create salmon/dennisr
# zfs create salmon/billj

# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
salmon                 168K   134G  28.5K  /salmon
salmon/billj          24.5K   134G  24.5K  /salmon/billj
salmon/dennisr        24.5K   134G  24.5K  /salmon/dennisr
salmon/kent           24.5K   134G  24.5K  /salmon/kent

Note how all four filesystems share the same pool space and all report 134 GB available. We’ll see how to set quotas and reserve space for filesystems later in this tutorial.

We can create arbitrary levels of filesystems, so you could create whole tree of filesystems inside /salmon/kent.

We can also see our filesystems using df (output trimmed for brevity):

# df -h
Filesystem             size   used  avail capacity  Mounted on
salmon                 134G    28K   134G     1%    /salmon
salmon/kent            134G    24K   134G     1%    /salmon/kent
salmon/dennisr         134G    24K   134G     1%    /salmon/dennisr
salmon/billj           134G    24K   134G     1%    /salmon/billj

You can remove filesystems with zfs destroy. User billj has stopped working on salmon, so let’s remove him:

# zfs destroy salmon/billj
# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
salmon                 138K   134G  28.5K  /salmon
salmon/dennisr        24.5K   134G  24.5K  /salmon/dennisr
salmon/kent           24.5K   134G  24.5K  /salmon/kent

Mount Points

It’s useful that ZFS automatically mounts your filesystem under the pool name, but this is often not what you want. Thankfully it’s very easy to change the properties of a ZFS filesystem, even when it’s mounted.

You can set the mount point of a ZFS filesystem using zfs set mountpoint. For example, if we want to move salmon under /projects directory:

# zfs set mountpoint=/projects/salmon salmon
# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
salmon                 142K   134G  27.5K  /projects/salmon
salmon/dennisr        24.5K   134G  24.5K  /projects/salmon/dennisr
salmon/kent           24.5K   134G  24.5K  /projects/salmon/kent

On Mac OS X you need to force an unmount of the filesyetem (using umount -f /Volumes/salmon) before changing the mount point as it will be in use by fseventsd. To mount it again after setting a new mount point use ‘zfs mount salmon’.

Mount points of filesystems are not limited to those of the pool as a whole, for example:

# zfs set mountpoint=/fishing salmon/kent
# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
salmon                 148K   134G  27.5K  /projects/salmon
salmon/dennisr        24.5K   134G  24.5K  /projects/salmon/dennisr
salmon/kent           24.5K   134G  24.5K  /fishing

To mount and unmount ZFS filesystems you use zfs mount and zfs unmount (Old school Unix users will be pleased to know ‘zfs umount’ works too). ZFS filesystems are entirely managed by ZFS by default, and don’t appear in /etc/vfstab. In a future tutorial we will look at using ‘legacy’ mount points to manage filesystems the traditional way.

For example (mount output trimmed for brevity):

# zfs unmount salmon/kent

# mount | grep salmon
/projects/salmon on salmon
/projects/salmon/dennisr on salmon/dennisr

# zfs mount salmon/kent

# mount | grep salmon
/projects/salmon on salmon 
/projects/salmon/dennisr on salmon/dennisr 
/fishing on salmon/kent

Managing ZFS Filesystem Properties

Other filesystem properties work in the same way as the mount point (which is itself a property). To get and set properties we use zfs get and zfs set. To see a list of all filesystem properties we can use ‘zfs get all’:

# zfs get all salmon/kent
NAME             PROPERTY       VALUE                      SOURCE
salmon/kent      type           filesystem                 -
salmon/kent      creation       Fri Apr  6 13:14 2007      -
salmon/kent      used           24.5K                      -
salmon/kent      available      134G                       -
salmon/kent      referenced     24.5K                      -
salmon/kent      compressratio  1.00x                      -
salmon/kent      mounted        yes                        -
salmon/kent      quota          none                       default
salmon/kent      reservation    none                       default
salmon/kent      recordsize     128K                       default
salmon/kent      mountpoint     /fishing                   local
salmon/kent      sharenfs       off                        default
salmon/kent      checksum       on                         default
salmon/kent      compression    off                        default
salmon/kent      atime          on                         default
salmon/kent      devices        on                         default
salmon/kent      exec           on                         default
salmon/kent      setuid         on                         default
salmon/kent      readonly       off                        default
salmon/kent      zoned          off                        default
salmon/kent      snapdir        hidden                     default
salmon/kent      aclmode        groupmask                  default
salmon/kent      aclinherit     secure                     default

The first set of properties, with a SOURCE of ‘-‘, are read only and give information on your filesystem; the rest of the properties can be set with ‘zfs set’. The SOURCE value shows where a property gets its value from, other than ‘-’ there are three sources for a property:

• default - the default ZFS value for this property
• local - the property is set directly on this filesystem
• inherited - the property is inherited from a parent filesystem

The mountpoint property is shown as from a local source, this is because we set the mountpoint for this filesystem above. We’ll see an example of an inherited property in the section on compression (below).

I’m going to look at three properties in this section: quota, reservation and compression (sharenfs will be covered in a future tutorial). You can read about the remaining properties in the Sun ZFS Administration Guide.

Quotas & Reservations

All the filesystems in a pool share the same disk space, This maximises flexibility and lets ZFS make best use of the resources, however it does allow one filesystem to use all the space. To manage space utilisation within a pool, filesystems can have quotas and reservations. A quota sets a limit on the pool space a filesystem can use. A reservation reserves part of the pool for the exclusive use of one filesystem.

To see how this works, let’s consider our existing pool:

# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
salmon                 148K   134G  26.5K  /projects/salmon
salmon/dennisr        24.5K   134G  24.5K  /projects/salmon/dennisr
salmon/kent           24.5K   134G  24.5K  /fishing

For example, let’s say we want to set a quota of 10 GB on dennisr and kent to ensure there’s space for other users to be added to salmon (If you are using disk files or small disks just substitute a suitable value, eg. quota=10M):

# zfs set quota=10G salmon/dennisr
# zfs set quota=10G salmon/kent

# zfs get quota salmon salmon/kent salmon/dennisr
NAME             PROPERTY       VALUE                      SOURCE
salmon           quota          none                       default
salmon/dennisr   quota          10G                        local
salmon/kent      quota          10G                        local

You can see how we used zfs get to retrieve a particular property for a set of filesystems. There are some useful options we can use with get:

• -r recursively gets the property for all child filesystems.
• -p reports exact values (e.g. 9437184 rather than 9M).
• -H omits header fields, making the output easier for scripts to parse.
• -o <fields> specify a list of fields you wish to get (avoids having to use awk or cut).

An example (excluding headers and not showing the source field):

# zfs get -rHp -oname,property,value quota salmon
salmon          quota   0       
salmon/dennisr  quota   10737418240
salmon/kent     quota   10737418240

As an example of reservations let’s add a new filesystem and reserve 1 GB of space for it. This ensures that however full the disk gets, when someone comes to use it there will be space.

# zfs create salmon/jeffb
# zfs set reservation=1G salmon/jeffb

# zfs get -r reservation salmon
NAME             PROPERTY       VALUE                      SOURCE
salmon           reservation    none                       default
salmon/dennisr   reservation    none                       default
salmon/jeffb     reservation    1G                         local
salmon/kent      reservation    none                       default

If we look at our list of filesystems with zfs list we can see the effect of the quotas and reservation:

# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
salmon                1.00G   133G  27.5K  /projects/salmon
salmon/dennisr        24.5K  10.0G  24.5K  /projects/salmon/dennisr
salmon/jeffb          24.5K   134G  24.5K  /projects/salmon/jeffb
salmon/kent           24.5K  10.0G  24.5K  /fishing

As expected the space available to salmon/dennisr and salmon/kent is now limited to 10 GB, but there appears to be no change to salmon/jeffb. However, if we look at the used space for salmon as a whole we can see this has risen to 1 GB. This space isn’t actually used, but because it has been reserved for salmon/jeffb it isn’t available to the rest of the pool. Reservations could lead you to over-estimate the spaced used in your pool. The df command always displays the actual usage, so can be handy in such situations.

Compression

ZFS has built-in support for compression. Not only does this save disk space, but it can actually improve performance on systems with plenty of CPU and highly compressible data, as it saves disk I/O. An obvious candidate for compression is a logs directory.

solaris unix zfs file system I

You need:

• An operating system with ZFS support:
  • Solaris 10 6/06 or later 
  • OpenSolaris (discontinued)
  • FreeBSD 7 or later 
  • Mac OS X with maczfs
  • Linux with ZFS kernel module 
• Root privileges (or a role with the appropriate ZFS rights profile)
• Some storage, either:
  • 512 MB of disk space on an existing partition
  • Four spare disks of the same size

Using Files

To use files on an existing filesystem, create four 128 MB files, eg.:

# mkfile 128m /home/ocean/disk1
# mkfile 128m /home/ocean/disk2
# mkfile 128m /home/ocean/disk3
# mkfile 128m /home/ocean/disk4

# ls -lh /home/ocean
total 1049152
-rw------T   1 root     root        128M Mar  7 19:48 disk1
-rw------T   1 root     root        128M Mar  7 19:48 disk2
-rw------T   1 root     root        128M Mar  7 19:48 disk3
-rw------T   1 root     root        128M Mar  7 19:48 disk4

Using Disks

To use real disks in the tutorial make a note of their names (eg. c2t1d0 or c1d0 under Solaris). You will be destroying all the partition information and data on these disks, so be sure they’re not needed.

In the examples I will be using files named disk1, disk2, disk3, and disk4; substitute your disks or files for them as appropriate.

ZFS Overview

The architecture of ZFS has three levels. One or more ZFS filesystems exist in a ZFS pool, which consists of one of more devices* (usually disks). Filesystems within a pool share its resources and are not restricted to a fixed size. Devices may be added to a pool while its still running: eg. to increase the size of a pool. New filesystems can be created within a pool without taking filesystems offline. ZFS supports filesystems snapshots and cloning existing filesystems. ZFS manages all aspects of the storage: volume management software (such as SVM or Veritas) is not needed.

*Technically a virtual device (vdev), see the zpool(1M) man page for more.

ZFS is managed with just two commands:

• zpool - Manages ZFS pools and the devices within them.
• zfs - Manages ZFS filesystems.

If you run either command with no options it gives you a handy options summary.

Pools

All ZFS filesystems live in a pool, so the first step is to create a pool. ZFS pools are administered using the zpool command.

Before creating new pools you should check for existing pools to avoid confusing them with your tutorial pools. You can check what pools exist with zpool list:

# zpool list
no pools available

NB. OpenSolaris now uses ZFS, so you will likely have an existing ZFS pool called syspool on this OS.

Single Disk Pool

The simplest pool consist of a single device. Pools are created using zpool create. We can create a single disk pool as follows (you must use the absolute path to the disk file):

# zpool create herring /home/ocean/disk1
# zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
herring                 123M   51.5K    123M     0%  ONLINE     -

No volume management, configuration, newfs or mounting is required. You now have a working pool complete with mounted ZFS filesystem under /herring (/Volumes/herring on Mac OS X - you can also see it mounted on your Mac desktop). We will learn about adjusting mount points in part 2 of the tutorial.

Create a file in the new filesystem:

# mkfile 32m /herring/foo
# ls -lh /herring/foo 
-rw------T   1 root     root         32M Mar  7 19:56 /herring/foo

# zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
herring                 123M   32.1M   90.9M    26%  ONLINE     -

The new file is using about a quarter of the pool capacity (indicated by the CAP value). NB. If you run the list command before ZFS has finished writing to the disk you will see lower USED and CAP values than shown above; wait a few moments and try again.

Now destroy your pool with zpool destroy:

# zpool destroy herring
# zpool list
no pools available

On Mac OS X you need to force an unmount of the filesyetem (using umount -f /Volumes/herring) before destroying it as it will be in use by fseventsd.

You will only receive a warning about destroying your pool if it’s in use. We’ll see in a later tutorial how you can recover a pool you’ve accidentally destroyed.

Mirrored Pool

A pool composed of a single disk doesn’t offer any redundancy. One method of providing redundancy is to use a mirrored pair of disk as a pool:

# zpool create trout mirror /home/ocean/disk1 /home/ocean/disk2

# zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
trout                   123M   51.5K    123M     0%  ONLINE     -

To see more detail about the pool use zpool status:

# zpool status trout
  pool: trout
 state: ONLINE
 scrub: none requested
config:
        NAME                          STATE     READ WRITE CKSUM
        trout                         ONLINE       0     0     0
          mirror                      ONLINE       0     0     0
            /home/ocean/disk1         ONLINE       0     0     0
            /home/ocean/disk2         ONLINE       0     0     0

errors: No known data errors

We can see our pool contains one mirror of two disks. Let’s create a file and see how USED changes:

# mkfile 32m /trout/foo

# zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
trout                   123M   32.1M   90.9M    26%  ONLINE     -

As before about a quarter of the disk has been used; but the data is now stored redundantly over two disks. Let’s test it by overwriting the first disk label with random data (if you are using real disks you could physically disable or remove a disk instead):

# dd if=/dev/random of=/home/ocean/disk1 bs=512 count=1

ZFS automatically checks for errors when it reads/writes files, but we can force a check with the zfs scrub command.

# zpool scrub trout

# zpool status
  pool: trout
 state: DEGRADED
status: One or more devices could not be used because the label is missing or
        invalid.  Sufficient replicas exist for the pool to continue
        functioning in a degraded state.
action: Replace the device using 'zpool replace'.
   see: http://www.sun.com/msg/ZFS-8000-4J
 scrub: scrub completed with 0 errors on Wed Mar  7 20:42:07 2007
config:
        NAME                          STATE     READ WRITE CKSUM
        trout                         DEGRADED     0     0     0
          mirror                      DEGRADED     0     0     0
            /home/ocean/disk1         UNAVAIL      0     0     0  corrupted data
            /home/ocean/disk2         ONLINE       0     0     0

errors: No known data errors

The disk we used dd on is showing as UNAVAIL with corrupted data, but no data errors are reported for the pool as a whole, and we can still read and write to the pool:

# mkfile 32m /trout/bar
# ls -l /trout/
total 131112
-rw------T   1 root     root     33554432 Mar  7 20:43 bar
-rw------T   1 root     root     33554432 Mar  7 20:35 foo
To maintain redundancy we should replace the broken disk with another. If you are using a physical disk you can use the zpool replace command (the zpool man page has details). However, in this file-based example I remove the disk file from the mirror and recreate it.

Devices are detached with zpool detach:

# zpool detach trout /home/ocean/disk1

# zpool status trout
  pool: trout
 state: ONLINE
 scrub: scrub completed with 0 errors on Wed Mar  7 20:42:07 2007
config:
        NAME                        STATE     READ WRITE CKSUM
        trout                       ONLINE       0     0     0
          /home/ocean/disk2         ONLINE       0     0     0

errors: No known data errors

# rm /home/ocean/disk1
# mkfile 128m /home/ocean/disk1

To attach another device we specify an existing device in the mirror to attach it to with zpool attach:

# zpool attach trout /home/ocean/disk2 /home/ocean/disk1

If you’re quick enough, after you attach the new disk you will see a resilver (remirroring) in progress with zpool status.

# zpool status trout
  pool: trout
 state: ONLINE
status: One or more devices is currently being resilvered.  The pool will
        continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
 scrub: resilver in progress, 69.10% done, 0h0m to go
config:
        NAME                          STATE     READ WRITE CKSUM
        trout                         ONLINE       0     0     0
          mirror                      ONLINE       0     0     0
            /home/ocean/disk2         ONLINE       0     0     0
            /home/ocean/disk1         ONLINE       0     0     0

errors: No known data errors

Once the resilver is complete, the pool is healthy again (you can also use ls to check the files are still there):

# zpool status trout
  pool: trout
 state: ONLINE
 scrub: resilver completed with 0 errors on Wed Mar  7 20:58:17 2007
config:
        NAME                          STATE     READ WRITE CKSUM
        trout                         ONLINE       0     0     0
          mirror                      ONLINE       0     0     0
            /home/ocean/disk2         ONLINE       0     0     0
            /home/ocean/disk1         ONLINE       0     0     0

errors: No known data errors

Adding to a Mirrored Pool

You can add disks to a pool without taking it offline. Let’s double the size of our trout pool:

# zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
trout                   123M   64.5M   58.5M    52%  ONLINE     -

# zpool add trout mirror /home/ocean/disk3 /home/ocean/disk4

# zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
trout                   246M   64.5M    181M    26%  ONLINE     -

This happens almost instantly, and the filesystem within the pool remains available. Looking at the status now shows the pool consists of two mirrors:

# zpool status trout
  pool: trout
 state: ONLINE
 scrub: resilver completed with 0 errors on Wed Mar  7 20:58:17 2007
config:
        NAME                          STATE     READ WRITE CKSUM
        trout                         ONLINE       0     0     0
          mirror                      ONLINE       0     0     0
            /home/ocean/disk2         ONLINE       0     0     0
            /home/ocean/disk1         ONLINE       0     0     0
          mirror                      ONLINE       0     0     0
            /home/ocean/disk3         ONLINE       0     0     0
            /home/ocean/disk4         ONLINE       0     0     0

errors: No known data errors

We can see where the data is currently written in our pool using zpool iostat -v:

zpool iostat -v trout
                                 capacity     operations    bandwidth
pool                           used  avail   read  write   read  write
----------------------------  -----  -----  -----  -----  -----  -----
trout                         64.5M   181M      0      0  13.7K    278
  mirror                      64.5M  58.5M      0      0  19.4K    394
    /home/ocean/disk2             -      -      0      0  20.6K  15.4K
    /home/ocean/disk1             -      -      0      0      0  20.4K
  mirror                          0   123M      0      0      0      0
    /home/ocean/disk3             -      -      0      0      0    768
    /home/ocean/disk4             -      -      0      0      0    768
----------------------------  -----  -----  -----  -----  -----  -----

All the data is currently written on the first mirror pair, and none on the second. This makes sense as the second pair of disks was added after the data was written. If we write some new data to the pool the new mirror will be used:

# mkfile 64m /trout/quuxx

# zpool iostat -v trout
                                 capacity     operations    bandwidth
pool                           used  avail   read  write   read  write
----------------------------  -----  -----  -----  -----  -----  -----
trout                          128M   118M      0      0  13.1K  13.6K
  mirror                      95.1M  27.9M      0      0  18.3K  9.29K
    /home/ocean/disk2             -      -      0      0  19.8K  21.2K
    /home/ocean/disk1             -      -      0      0      0  28.2K
  mirror                      33.2M  89.8M      0      0      0  10.4K
    /home/ocean/disk3             -      -      0      0      0  11.1K
    /home/ocean/disk4             -      -      0      0      0  11.1K
----------------------------  -----  -----  -----  -----  -----  -----

Note how a little more of the data has been written to the new mirror than the old: ZFS tries to make best use of all the resources in the pool.