Difference between revisions of "Experimenting with ZFS"

From ArchWiki
Jump to: navigation, search
m (Install the ZFS Family of Packages)
m (RAIDZ1)
Line 19: Line 19:
  
 
=== RAIDZ1 ===
 
=== RAIDZ1 ===
The minimum number of drives for a RAIDZ1 is three.  In his [https://pthree.org/2012/12/13/zfs-administration-part-viii-zpool-best-practices-and-caveats/ blog] on ZFS (an excellent read), Aaron Toponce recommends following the "power of two plus parity" recommendation.  This is for storage space efficiency and hitting the "sweet spot" in performance.  For RAIDZ-1, use three (2+1), five (4+1), or nine (8+1) disks. This example will use the most simplistic set of (2+1).
+
The minimum number of drives for a RAIDZ1 is three.  It's best to follow the "power of two plus parity" recommendation.  This is for storage space efficiency and hitting the "sweet spot" in performance.  For RAIDZ-1, use three (2+1), five (4+1), or nine (8+1) disks. This example will use the most simplistic set of (2+1).
  
 
Create three x 2G files to serve as virtual hardrives:
 
Create three x 2G files to serve as virtual hardrives:

Revision as of 23:36, 22 October 2013

Summary help replacing me
This article covers some basic tasks and usage of ZFS. It differs from the parent article linked below somwhat in that the examples herein are demonstrated on a zpool built from virtual disks. So long as users do not place any critical data on the resulting zpool, they are free to experiment without fear of actual data loss.
Related
ZFS

As the article summary notes, the examples in this article are shown with a set of virtual discs known in ZFS terms as VDEVs. Users may create their VDEVs either on an existing physical disk or in tmpfs (RAMdisk) depending on the amount of free memory on the system.

Note: Using a file as a VDEV is a great method to play with ZFS but isn't viable strategy for storing "real" data.

Install the ZFS Family of Packages

Due to differences in licencing, ZFS bins and kernel modules are easily distributed from source, but no-so-easily packaged as pre-compiled sets. The requisite packages are available in the AUR and in an unofficial repo. Details are provided on the ZFS Installation article.

Creating and Destroying Zpools

Management of ZFS is pretty simplistic with only two utils needed:

  • /usr/bin/zpool
  • /usr/bin/zfs

RAIDZ1

The minimum number of drives for a RAIDZ1 is three. It's best to follow the "power of two plus parity" recommendation. This is for storage space efficiency and hitting the "sweet spot" in performance. For RAIDZ-1, use three (2+1), five (4+1), or nine (8+1) disks. This example will use the most simplistic set of (2+1).

Create three x 2G files to serve as virtual hardrives:

$ for i in {1..3}; do truncate -s 2G /scratch/$i.img; done

Assemble the RAIDZ1:

# zpool create myraidz1 raidz1 /scratch/1.img /scratch/2.img /scratch/3.img

Notice that a 3.91G zpool has been created and mounted for us:

# zfs list
 NAME   USED  AVAIL  REFER  MOUNTPOINT
 test   139K  3.91G  38.6K  /myraidz1

The status of the device can be queried:

# zpool status myraidz1
  pool: myraidz1
 state: ONLINE
  scan: none requested
config:

	NAME                STATE     READ WRITE CKSUM
	myraidz1            ONLINE       0     0     0
	  raidz1-0          ONLINE       0     0     0
	    /scratch/1.img  ONLINE       0     0     0
	    /scratch/2.img  ONLINE       0     0     0
	    /scratch/3.img  ONLINE       0     0     0

errors: No known data errors

To destroy a zpool:

# zpool destroy myraidz1

RAIDZ2 and RAIDZ3

Higher level ZRAIDs can be assembled in a like fashion by adjusting the for statement to create the image files, by specifying "raidz2" or "raidz3" in the creation step, and by appending the additional image files to the creation step.

Summarizing Toponce's guidance:

  • RAIDZ2 should use four (2+2), six (4+2), ten (8+2), or eighteen (16+2) disks.
  • RAIDZ3 should use five (2+3), seven (4+3), eleven (8+3), or nineteen (16+3) disks.

Displaying and Setting Properties

Without specifying them in the creation step, users can set properties of their zpools at any time after its creation using /usr/bin/zfs.

Show Properties

To see the current properties of a given zpool:

# zfs get all myraidz1
NAME      PROPERTY              VALUE                  SOURCE
myraidz1  type                  filesystem             -
myraidz1  creation              Sun Oct 20  8:46 2013  -
myraidz1  used                  139K                   -
myraidz1  available             3.91G                  -
myraidz1  referenced            38.6K                  -
myraidz1  compressratio         1.00x                  -
myraidz1  mounted               yes                    -
myraidz1  quota                 none                   default
myraidz1  reservation           none                   default
myraidz1  recordsize            128K                   default
myraidz1  mountpoint            /myraidz1              default
myraidz1  sharenfs              off                    default
myraidz1  checksum              on                     default
myraidz1  compression           off                    default
myraidz1  atime                 on                     default
myraidz1  devices               on                     default
myraidz1  exec                  on                     default
myraidz1  setuid                on                     default
myraidz1  readonly              off                    default
myraidz1  zoned                 off                    default
myraidz1  snapdir               hidden                 default
myraidz1  aclinherit            restricted             default
myraidz1  canmount              on                     default
myraidz1  xattr                 on                     default
myraidz1  copies                1                      default
myraidz1  version               5                      -
myraidz1  utf8only              off                    -
myraidz1  normalization         none                   -
myraidz1  casesensitivity       sensitive              -
myraidz1  vscan                 off                    default
myraidz1  nbmand                off                    default
myraidz1  sharesmb              off                    default
myraidz1  refquota              none                   default
myraidz1  refreservation        none                   default
myraidz1  primarycache          all                    default
myraidz1  secondarycache        all                    default
myraidz1  usedbysnapshots       0                      -
myraidz1  usedbydataset         38.6K                  -
myraidz1  usedbychildren        99.9K                  -
myraidz1  usedbyrefreservation  0                      -
myraidz1  logbias               latency                default
myraidz1  dedup                 off                    default
myraidz1  mlslabel              none                   default
myraidz1  sync                  standard               default
myraidz1  refcompressratio      1.00x                  -
myraidz1  written               38.6K                  -
myraidz1  snapdev               hidden                 default

Modify properties

Disable the recording of access time in the zpool:

# zfs set atime=off myraidz1

Verify that the property has been set on the zpool:

# zfs get atime
NAME  PROPERTY     VALUE     SOURCE
myraidz1  atime        off       local
Tip: This option like many others can be toggled off when creating the zpool as well by appending the following to the creation step: -O atime-off

Add Content to the Zpool and Query Compression Performance

Fill the zpool with files. For this example, first enable compression. ZFS uses many compression types, including, lzjb, gzip, gzip-N, zle, and lz4. Using a setting of simply 'on' will call the default algorithm (lzjb) but lz4 is a nice alternative. See the zfs man page for more.

# zfs set compression=lz4 myraidz1

In this example, the linux source tarball is copied over and since lz4 compression has been enabled on the zpool, the corresponding compression ratio can be queried as well.

$ wget https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.11.tar.xz
$ tar xJf linux-3.11.tar.xz -C /myraidz1 

To see the compression ratio achieved:

# zfs get compressratio
NAME      PROPERTY       VALUE  SOURCE
myraidz1  compressratio  2.32x  -

Simulate a Disk Failure and Rebuild the Zpool

To simulate catastrophic disk failure (i.e. one of the HDDs in the zpool stops functioning), zero out one of the VDEVs.

$ dd if=/dev/zero of=/scratch/2.img bs=4M count=1 2>/dev/null

Since we used a blocksize (bs) of 4M, the once 2G image file is now a mere 4M:

$ ls -lh /scratch 
total 317M
-rw-r--r-- 1 facade users 2.0G Oct 20 09:13 1.img
-rw-r--r-- 1 facade users 4.0M Oct 20 09:09 2.img
-rw-r--r-- 1 facade users 2.0G Oct 20 09:13 3.img

The zpool remains online despite the corruption. Note that if a physical disc does fail, dmesg and related logs would be full of errors. To detect when damage occurs, users must execute a scrub operation.

# zpool scrub myraidz1

Depending on the size and speed of the underlying media as well as the amount of data in the zpool, the scrub may take hours to complete. The status of the scrub can be queried:

# zpool status myraidz1
  pool: myraidz1
 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://zfsonlinux.org/msg/ZFS-8000-4J
  scan: scrub repaired 0 in 0h0m with 0 errors on Sun Oct 20 09:13:39 2013
config:

	NAME                STATE     READ WRITE CKSUM
	myraidz1            DEGRADED     0     0     0
	  raidz1-0          DEGRADED     0     0     0
	    /scratch/1.img  ONLINE       0     0     0
	    /scratch/2.img  UNAVAIL      0     0     0  corrupted data
	    /scratch/3.img  ONLINE       0     0     0

errors: No known data errors

Since we zeroed out one of our VDEVs, let's simulate adding a new 2G HDD by creating a new image file and adding it to the zpool:

$ truncate -s 2G /scratch/new.img
# zpool replace myraidz1 /scratch/2.img /scratch/new.img

Upon replacing the VDEV with a new one, zpool rebuilds the data from the data and parity info in the remaining two good VDEVs. Check the status of this process:

# zpool status myraidz1 
  pool: myraidz1
 state: ONLINE
  scan: resilvered 117M in 0h0m with 0 errors on Sun Oct 20 09:21:22 2013
config:

	NAME                  STATE     READ WRITE CKSUM
	myraidz1              ONLINE       0     0     0
	  raidz1-0            ONLINE       0     0     0
	    /scratch/1.img    ONLINE       0     0     0
	    /scratch/new.img  ONLINE       0     0     0
	    /scratch/3.img    ONLINE       0     0     0

errors: No known data errors

Snapshots and Recovering Deleted Files

Since ZFS is a copy-on-write filesystem, every file exists the second it is written. Saving changes to the very same file actually creates another copy of that file (plus the changes made). Snapshots can take advantage of this fact and allow users access to older versions of files provided a snapshot has been taken.

Note: When using snapshots, many Linux programs that report on filesystem space such as df will report inaccurate results due to the unique way snapshots are used on ZFS. The output of /usr/bin/zfs list will deliver an accurate report of the amount of available and free space on the zpool.


To keep this simple, we will create a dataset within the zpool and snapshot it. Snapshots can be taken either of the entire zpool or of a dataset within the pool. They differ only in their naming conventions:

Snapshot Target Snapshot Name
Entire zpool myzpool@snapshot-name
Dataset myzpool/dataset@snapshot-name

Make a new data set and take ownership of it.

# zfs create myzpool/docs
# chown facade:users /myzpool/docs
Note: The lack of a proceeding / in the create command is intentional, not a typo!

Time 0

Add some files to the new dataset (/myzpool/docs):

$ wget -O /myzpool/docs/Moby_Dick.txt  http://www.gutenberg.org/ebooks/2701.txt.utf-8
$ wget -O /myzpool/docs/War_and_Peace.txt http://www.gutenberg.org/ebooks/2600.txt.utf-8
$ wget -O /myzpool/docs/Beowulf.txt http://www.gutenberg.org/ebooks/16328.txt.utf-8
# zfs list
NAME           USED  AVAIL  REFER  MOUNTPOINT
myzpool       5.06M  3.91G  40.0K  /myzpool
myzpool/docs  4.92M  3.91G  4.92M  /myzpool/docs

This is showing that we have 4.92M of data used by our books in /myzpool/docs.

Time +1

Now take a snapshot of the dataset:

# zfs snapshot myzpool/docs@001

Again run the list command:

# zfs list
NAME           USED  AVAIL  REFER  MOUNTPOINT
myzpool       5.07M  3.91G  40.0K  /myzpool
myzpool/docs  4.92M  3.91G  4.92M  /myzpool/docs

Note that the size in the USED col did not change showing that the snapshot take up no space in the zpool since nothing has changed in these three files.

We can list out the snapshots like so and again confirm the snapshot is taking up no space, but instead refers to files from the originals that take up, 4.92M (their original size):

# zfs list -t snapshot
NAME               USED  AVAIL  REFER  MOUNTPOINT
myzpool/docs@001      0      -  4.92M  -

Time +2

Now let's add some additional content and create a new snapshot:

$ wget -O /myzpool/docs/Les_Mis.txt http://www.gutenberg.org/ebooks/135.txt.utf-8
# zfs snapshot myzpool/docs@002

Generate the new list to see how the space has changed:

# zfs list -t snapshot
NAME               USED  AVAIL  REFER  MOUNTPOINT
myzpool/docs@001  25.3K      -  4.92M  -
myzpool/docs@002      0      -  8.17M  -

Here we can see that the 001 snapshot takes up 25.3K of metadata and still points to the original 4.92M of data, and the new snapshot takes-up no space and refers to a total of 8.17M.

Time +3

Now let's simulate an accidental overwrite of a file and subsequent data loss:

$ echo "this book sucks" > /myzpool/docs/War_and_Peace.txt

Again, take another snapshot:

# zfs snapshot myzpool/docs@003

Now list out the snapshots and notice the amount of referred to decreased by about 3.1M:

# zfs list -t snapshot
NAME               USED  AVAIL  REFER  MOUNTPOINT
myzpool/docs@001  25.3K      -  4.92M  -
myzpool/docs@002  25.5K      -  8.17M  -
myzpool/docs@003      0      -  5.04M  -

We can easily recover from this situation by looking inside one or both of our older snapshots for good copy of the file. ZFS stores its snapshots in a hidden directory under the zpool: /myzpool/files/.zfs/snapshot:

$ ls -l /myzpool/docs/.zfs/snapshot
total 0
dr-xr-xr-x 1 root root 0 Oct 20 16:09 001
dr-xr-xr-x 1 root root 0 Oct 20 16:09 002
dr-xr-xr-x 1 root root 0 Oct 20 16:09 003

We can copy a good version of the book back out from any of our snapshots to any location on or off the zpool:

% cp /myzpool/docs/.zfs/snapshot/002/War_and_Peace.txt /myzpool/docs
Note: Using <TAB> for autocompletion will not work by default but can be changed by modifying the snapdir property on the pool or dataset.
# zfs set snapdir=visible myzpool/docs

As a side effect of this change, however, any snapshot will be listed as datasets to the linux kernel if the user has browsed into them:

Example:

$ df -h | grep myzpool
myzpool           4.0G     0  4.0G   0% /myzpool
myzpool/docs      4.0G  5.0M  4.0G   1% /myzpool/docs

Now enter a snapshot dir or two:

$ cd /myzpool/docs/.zfs/snapshot/001
$ cd /myzpool/docs/.zfs/snapshot/002

Repeat the df command:

$ df -h | grep myzpool
myzpool           4.0G     0  4.0G   0% /myzpool
myzpool/docs      4.0G  5.0M  4.0G   1% /myzpool/docs
myzpool/docs@001  4.0G  4.9M  4.0G   1% /myzpool/docs/.zfs/snapshot/001
myzpool/docs@002  4.0G  8.2M  4.0G   1% /myzpool/docs/.zfs/snapshot/002
Note: This effect is reversible if the zpool is taken offline and then remounted.

For example:

# zpool export myzpool
# zpool import -d /scratch/ myzpool
$ df -h | grep myzpool
myzpool         4.0G     0  4.0G   0% /myzpool
myzpool/docs    4.0G  5.0M  4.0G   1% /myzpool/docs

Time +4

Now that everything is back to normal, we can create another snapshot of this state:

# zfs snapshot myzpool/docs@004

And the list now becomes:

# zfs list -t snapshot
NAME               USED  AVAIL  REFER  MOUNTPOINT
myzpool/docs@001  25.3K      -  4.92M  -
myzpool/docs@002  25.5K      -  8.17M  -
myzpool/docs@003   155K      -  5.04M  -
myzpool/docs@004      0      -  8.17M  -

Deleting Snapshots

The limit to the number of snapshots users can save is 2^64. User can delete a snapshot like so:

# zfs destroy myzpool/docs@001
# zfs list -t snapshot
NAME               USED  AVAIL  REFER  MOUNTPOINT
myzpool/docs@002  3.28M      -  8.17M  -
myzpool/docs@003   155K      -  5.04M  -
myzpool/docs@004      0      -  8.17M  -