Zfs raid level openzfs11/10/2022 recordsize=4K or 8K), these workloads benefit greatly from compression. databases) do use 4KB or 8KB logical block sizes (i.e. The primary flaw with this recommendation is that it assumes that you are using small blocks whose size is a power of 2. These people would claim that for example, a 9-wide (2^3+1) RAIDZ1 is better than 8-wide or 10-wide. Therefore, RAID-Z requires a bit more space for parity and overhead than RAID-4/5/6.Ī misunderstanding of this overhead, has caused some people to recommend using "(2^n)+p" disks, where p is the number of parity "disks" (i.e. the light blue block at left in rows 8-9 with 1 parity + 2 data + 1 padding). #Zfs raid level openzfs plus#too small to fit even a single sector of data plus p parity sectors - e.g. #Zfs raid level openzfs free#RAID-Z also requires that each allocation be a multiple of (p+1), so that when it is freed it does not leave a free segment which is too small to be used (i.e. Note that if there are several blocks sharing what would traditionally be thought of as a single "stripe", there will be multiple parity blocks in the "stripe". A 11-sector block will use 1 parity + 4 data + 1 parity + 4 data + 1 parity + 3 data (e.g. A 3-sector block will use one sector of parity plus 3 sectors of data (e.g. RAID-Z block layout RAID-Z parity information is associated with each block, rather than with specific stripes as with RAID-4/5/6. with RAIDZ-2, going from 7 to 12 disks will reduce the amount of parity information from 40% to 20%). Streaming write performance is proportional to space efficiency.įor space efficiency, typically doubling the number of "data" disks will halve the amount of parity per MB of data (e.g. Note that streaming read performance is independent of RAIDZ configuration, because only the data is read. To double your read IOPS, you would need to halve the number of "data" disks in the RAID-Z group (e.g. To double your write IOPS, you would need to halve the number of disks in the RAID-Z group. (In certain exceptional cases, use at least 5, 6, or 11 disks (for RAIDZ-1, 2, or 3 respectively) - see below for more details.) When trading off between these concerns, it is useful to know how much it helps to vary the parameters.įor performance on random IOPS, each RAID-Z group has approximately the performance of a single disk in the group. Wider stripes never hurts space efficiency. Space used by parity information for RAIDZ1įor best space efficiency, use a large number of disks in each RAID-Z group. This can tolerate any 3 whole shelves dying (or any 1 whole shelf dying plus any 2 other disks dying). E.g, if you have 10 shelves of 24 disks each, you could use 24 RAIDZ3 groups, each with 10 disks - one from each shelf. RAIDZ3 instead of RAIDZ1), and architect your groups to match your storage hardware. For even better performance, consider using mirroring.įor best reliability, use more parity (e.g. This is because RAID-Z spreads each logical block across all the devices (similar to RAID-3, in contrast with RAID-4/5/6). E.g, 3-wide RAIDZ1, 6-wide RAIDZ2, or 9-wide RAIDZ3 (all of which use a." of total storage for parity, in the ideal case of using large blocks). Trying to optimize your RAID-Z stripe width based on exact numbers is irrelevant in nearly all cases.įor best performance on random IOPS, use a small number of disks in each RAID-Z group. If you need more usable space, use more disks per stripe. TL DR: Choose a RAID-Z stripe width based on your IOPS needs and the amount of space you are willing to devote to parity information. If you need more IOPS, use fewer disks per stripe. To do so, let's start by looking at what concerns play into choice of group width. In general, this is a great aspect of the ZFS community, but I'd like to take the opportunity to address one piece of misinformed advice about how many disks to put in each RAID-Z group (terminology: "zpool create tank raidz1 A1 A2 A3 A4 raidz1 B1 B2 B3 B4" has 2 RAIDZ groups or "vdevs", each of which has 4 disks or is "4-wide"). #Zfs raid level openzfs how to#The popularity of OpenZFS has spawned a great community of users, sysadmins, architects and developers, contributing a wealth of advice, tips and tricks, and rules of thumb on how to configure ZFS.
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