Support #448: Experimenting with ZFS - GNU/Linux Administration - ALT Project Management

Support #448

Experimenting with ZFS

Added by Daniel Curtis over 9 years ago. Updated about 9 years ago.

Status:

Closed

Priority:

Normal

Assignee:

Daniel Curtis

Category:

Network Attached Storage

Target version:

Start date:

09/01/2014

Due date:

% Done:

100%

Estimated time:

2.00 h

Spent time:

3.50 h

Description

Table of contents
Install the ZFS Family of Packages
Creating and Destroying Zpools
Displaying and Setting Properties
- Show Properties
- Modify properties
Add Content to the Zpool and Query Compression Performance
Simulate a Disk Failure and Rebuild the Zpool
Snapshots and Recovering Deleted Files
Resources

This article covers some basic tasks and usage of ZFS. It differs from the main article ZFS somewhat 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.

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.

Creating and Destroying Zpools¶

Management of ZFS is pretty simplistic with only two utils needed:
```
/usr/bin/zpool
/usr/bin/zfs
```

Mirror¶

For zpools with just two drives, it is recommended to use ZFS in mirror mode which functions like a RAID0 mirroring the data. While this configuration is fine, higher RAIDZ levels are recommended.

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 3 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 zpool 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

Example output:

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

The status of the device can be queried:

zpool status zpool

Example output:

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

To destroy a zpool:
```
zpool destroy zpool
```

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 zpool

Example output:

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

Modify properties¶

Disable the recording of access time in the zpool:
```
zfs set atime=off zpool
```

Verify that the property has been set on the zpool:

zfs get atime

Example output:

NAME  PROPERTY     VALUE     SOURCE
zpool  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.
```
zfs set compression=lz4 zpool
```
NOTE: See the zfs man page for more.

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

Example output:

NAME      PROPERTY       VALUE  SOURCE
zpool  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

Example output:

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

Example output:

  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

Example output:

  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

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: zpool@snapshot-name
- Dataset: zpool/dataset@snapshot-name

Make a new data set and take ownership of it.

zfs create zpool/docs
chown facade:users /zpool/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 (/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

Now check the status of the datasets:

zfs list

Example output:

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
```
- Example output:
```
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
```
- Example output:
```
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

Example output:

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

Example output:

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
```
- Example output:
```
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
```
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 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

Example output:

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

NOTE: 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

Example output:

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

Example output:

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  -

Deleting Snapshots¶

The limit to the number of snapshots users can save is 2^64.

Snapshots can be deleted like so:

zfs destroy zpool/docs@001
zfs list -t snapshot

Example output:

NAME               USED  AVAIL  REFER  MOUNTPOINT
zpool/docs@002  3.28M      -  8.17M  -
zpool/docs@003   155K      -  5.04M  -
zpool/docs@004      0      -  8.17M  -

Resources¶

https://wiki.archlinux.org/index.php/Experimenting_with_ZFS

Updated by Daniel Curtis over 9 years ago

Status changed from New to Resolved
% Done changed from 0 to 100

Updated by Daniel Curtis over 9 years ago

Description updated (diff)

Updated by Daniel Curtis over 9 years ago

Status changed from Resolved to Closed

Updated by Daniel Curtis about 9 years ago

Description updated (diff)
Category set to Network Attached Storage

Updated by Daniel Curtis about 9 years ago

Description updated (diff)

Updated by Daniel Curtis about 9 years ago

Description updated (diff)

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GNet Solutions » GNU/Linux Administration

Support #448

Experimenting with ZFS

Install the ZFS Family of Packages¶

Creating and Destroying Zpools¶

Mirror¶

RAIDZ1¶

RAIDZ2 and RAIDZ3¶

Displaying and Setting Properties¶

Show Properties¶

Modify properties¶

Add Content to the Zpool and Query Compression Performance¶

Simulate a Disk Failure and Rebuild the Zpool¶

Snapshots and Recovering Deleted Files¶

Time 0¶

Time +1¶

Time +2¶

Time +3¶

Time +4¶

Deleting Snapshots¶

Resources¶

Updated by Daniel Curtis over 9 years ago

Updated by Daniel Curtis over 9 years ago

Updated by Daniel Curtis over 9 years ago

Updated by Daniel Curtis about 9 years ago

Updated by Daniel Curtis about 9 years ago

Updated by Daniel Curtis about 9 years ago