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Revision as of 13:26, 20 October 2013
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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.
Contents
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. Read and follow to the ZFS#Installation section to build and install the needed bins/modules.
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. In his 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).
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
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
Snapshotting
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