Support #448
Experimenting with ZFS
Description
- Table of contents
- Install the ZFS Family of Packages
- Creating and Destroying Zpools
- Displaying and Setting 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
- Example output:
- 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
- Example output:
- 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
- Example output:
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
- Example output:
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 -
- Example output:
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
- Example output:
- 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
- Example output:
- 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
- Example output:
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
- Example output:
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.
- Example output:
- 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 -
- Example output:
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 -
- Example output:
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 -
- Example output:
- 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
- Example output:
- 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
- Example output:
- 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
- Example output:
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 -
- Example output:
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 -