ZFS/仮想ディスク

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この記事では ZFS の基本的な使い方を説明しています。ZFS のメインの記事とは違って、このページでは仮想ディスクから作成した zpool を使用する例を紹介します。作成した zpool に重要なデータを置かなければ、データを消失することを恐れずに実験ができます。

ZFS は仮想ディスクのことを VDEV と呼称します。VDEV は既存の物理ディスクに作成したり、あるいは空きメモリ容量に応じて tmpfs (RAM ディスク) に作成できます。

ノート: ZFS で遊ぶときはファイルを VDEV として使うのも悪くないですが、実際のデータを保存する場合は実用的ではありません。

ZFS パッケージのインストール

ライセンスの違いにより、ZFS のバイナリやカーネルモジュールをコンパイル済みのパッケージとして配布することはできません。必要なパッケージは AUR や非公式リポジトリに存在します。詳しくは ZFS#インストールを見てください。

Zpool の作成と削除

ZFS を管理するときは以下の2つのユーティリティを使用します:

  • /usr/bin/zpool
  • /usr/bin/zfs

ミラー

Zpool を2つのディスクで使用して冗長性をもたせたい場合、RAID1 によるデータのミラーリングのように ZFS を mirror モードで使用することを推奨します。ミラーリングは RAID-Z の代わりとして使うこともできます。仮想ディスクのミラーリングに関する詳細は こちら を参照。

RAIDZ1

RAIDZ1 を組むのに必要なドライブは最低でも3台です。「2の冪乗+パリティ」の台数を用意するのがベストです。ストレージ領域を効率的に使用して最適なパフォーマンスを得ることができます。RAIDZ-1 の場合、3台 (2+1) 5台 (4+1) 9台 (8+1) のディスクを使います。以下の例では最もシンプルな (2+1) のセットを使います。

仮想ハードドライブとして 2GB のファイルを3つ作成:

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

RAIDZ1 を構築:

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

3.91GB の zpool が作成・マウントされます:

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

デバイスの状態を確認:

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

	NAME                STATE     READ WRITE CKSUM
	   zpool            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

zpool を削除するには:

# zpool destroy zpool

RAIDZ2 と 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.

Linear Span

This setup is for a JBOD, good for 3 or less drives normally, where space is still a concern and your not ready to move to full features of ZFS yet because of it. RaidZ will be your better bet once you achieve enough space to satisfy, since this setup is NOT taking advantage of the full features of ZFS, but has its roots safely set in a beginning array that will suffice for years until you build up your hard drive collection.

Assemble the Linear Span:

# zpool create san /dev/sdd /dev/sde /dev/sdf
# zpool status san
  pool: san
 state: ONLINE
  scan: scrub repaired 0 in 4h22m with 0 errors on Fri Aug 28 23:52:55 2015
config:

        NAME        STATE     READ WRITE CKSUM
        san         ONLINE       0     0     0
          sde       ONLINE       0     0     0
          sdd       ONLINE       0     0     0
          sdf       ONLINE       0     0     0

errors: No known data errors

データセットの作成と削除

An example creating child datasets and using compression:

  • create the datasets
# zfs create -p -o compression=on san/vault/falcon/snapshots
# zfs create -o compression=on san/vault/falcon/version
# zfs create -p -o compression=on san/vault/redtail/c/Users
  • now list the datasets (this was a linear span)
$ zfs list

Note, there is a huge advantage(file deletion) for making a 3 level dataset. If you have large amounts of data, by separating by datasets, its easier to destroy a dataset than to try and wait for recursive file removal to complete.

プロパティの確認と設定

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

プロパティの表示

zpool の現在のプロパティを確認するには:

# zfs get all zpool

プロパティの変更

zpool のアクセス日時の記録を無効化するには:

# zfs set atime=off zpool

zpool に設定されたプロパティを確認:

# zfs get atime
NAME  PROPERTY     VALUE     SOURCE
zpool  atime        off       local
ヒント: 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

Zpool にコンテンツを追加して圧縮性能を確認

zpool にファイルを保存してください。まずは圧縮を有効化します。ZFS では多数の圧縮アルゴリズムが使用できます: lzjb, gzip, gzip-N, zle, lz4 など。単純に 'on' に設定した場合はデフォルトのアルゴリズム (lzjb) が使われますが lz4 のほうが高速です。詳しくは zfs の man ページを見てください。

# zfs set compression=lz4 zpool

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 /zpool 

To see the compression ratio achieved:

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

ディスク故障をシミュレートして Zpool を再生成

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 zpool

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

	NAME                  STATE     READ WRITE CKSUM
	   zpool              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

スナップショットと削除済みファイルの復元

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.

ノート: 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:

スナップショットの対象 スナップショットの名前
zpool 全体 zpool@snapshot-name
データセット zpool/dataset@snapshot-name

Make a new data set and take ownership of it.

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

Time 0

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

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

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

Time +1

Now take a snapshot of the dataset:

# zfs snapshot zpool/docs@001

Again run the list command:

# zfs list
NAME           USED  AVAIL  REFER  MOUNTPOINT
zpool       5.07M  3.91G  40.0K  /zpool
zpool/docs  4.92M  3.91G  4.92M  /zpool/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
zpool/docs@001      0      -  4.92M  -

Time +2

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

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

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

# zfs list -t snapshot
NAME               USED  AVAIL  REFER  MOUNTPOINT
zpool/docs@001  25.3K      -  4.92M  -
zpool/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" > /zpool/docs/War_and_Peace.txt

Again, take another snapshot:

# zfs snapshot zpool/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
zpool/docs@001  25.3K      -  4.92M  -
zpool/docs@002  25.5K      -  8.17M  -
zpool/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: /zpool/files/.zfs/snapshot:

$ ls -l /zpool/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 /zpool/docs/.zfs/snapshot/002/War_and_Peace.txt /zpool/docs
ノート: 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 zpool/docs

Now enter a snapshot dir or two:

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

Repeat the df command:

$ df -h | grep zpool
zpool           4.0G     0  4.0G   0% /zpool
zpool/docs      4.0G  5.0M  4.0G   1% /zpool/docs
zpool/docs@001  4.0G  4.9M  4.0G   1% /zpool/docs/.zfs/snapshot/001
zpool/docs@002  4.0G  8.2M  4.0G   1% /zpool/docs/.zfs/snapshot/002
ノート: Seeing each dir under .zfs the user enters is reversible if the zpool is taken offline and then remounted or if the server is rebooted.

For example:

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

Time +4

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

# zfs snapshot zpool/docs@004

And the list now becomes:

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

スナップショットの確認

Note, this simple but important command is missing frequently from other articles on the subject, so its worth mention.

To list any snapshots on your system, run the following command

$ zfs list -t snapshot

スナップショットの削除

ユーザーが保存できるスナップショットの数は 2^64 までに制限されています。スナップショットは以下のようにして削除できます:

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

トラブルシューティング

If your system is not configured to load the zfs pool upon boot, or for whatever reason you want to manually remove and add back the pool, or if you have lost your pool completely, a convenient way is to use import/export.

If your pool was named <pool>

# zpool import pool

If you have any problems accessing your pool at any time, try export and reimport.

# zfs export pool
# zfs import pool