Flux»Tech


ZFS Tutorial Part 1

Learning to use ZFS, Sun’s new filesystem. Last updated 2013-01-11.

For the lastest version of this tutorial see my new blog post: ZFS Tutorial Part 1 (updated for 2016).

ZFS is an open source filesystem used in Solaris 10, with growing support from other operating systems. This series of tutorials shows you how to use ZFS with simple hands-on examples that require a minimum of resources.

In this tutorial I hope to give you a brief overview of ZFS and show you how to manage ZFS pools, the foundation of ZFS. In subsequent parts will we look at ZFS filesystems in more depth.

Let your hook be always cast; in the pool where you least expect it, there will be a fish. – Ovid

Getting Started

You need:

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:

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.

That’s it for part 1. In part 2 we will look at managing ZFS filesystems themselves and creating multiple filesystems within a pool. We’ll create a new pool for part 2, so feel free to destroy the trout pool.

If you want to learn more about the theory behind ZFS and find reference material have a look at ZFS Administration Guide, OpenSolaris ZFS, ZFS BigAdmin and ZFS Best Practices.


Tags: Filesystem, Solaris, Tutorial, ZFS