ZFS RAIDZ stripe width, or: How I Learned to Stop Worrying and Love RAIDZ

Что: 1faf1c4bdfe1748f262d6161ef40a5aff09d6ba7

Когда: 2017-08-27 12:05:28+03:00

Темы: zfs

https://www.delphix.com/blog/delphix-engineering/zfs-raidz-stripe-width-or-how-i-learned-stop-worrying-and-love-raidz
Очень хорошая и полезная статья для лучшего понимания устройства ZFS
RAIDZ и лучшего расчёта его конфигурации. Если статья пропадёт (а ссылок
по поводу ZFS очень много уже мёртвых), то будет потерей -- поэтому
привожу сюда её, чтобы локально на жёстком диске перманентно сохранить.

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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. 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"). To do so, let's start by
looking at what concerns play into choice of group width.

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. If you need more usable
space, use more disks per stripe. Trying to optimize your RAID-Z stripe
width based on exact numbers is irrelevant in nearly all cases.

For best performance on random IOPS, use a small number of disks in each
RAID-Z group. 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). This is because RAID-Z spreads each logical block
across all the devices (similar to RAID-3, in contrast with RAID-4/5/6).
For even better performance, consider using mirroring.

For best reliability, use more parity (e.g. RAIDZ3 instead of RAIDZ1),
and architect your groups to match your storage hardware. 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. This can tolerate any 3 whole
shelves dying (or any 1 whole shelf dying plus any 2 other disks dying).

For best space efficiency, use a large number of disks in each RAID-Z
group. Wider stripes never hurts space efficiency. (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.

For performance on random IOPS, each RAID-Z group has approximately the
performance of a single disk in the group. To double your write IOPS,
you would need to halve the number of disks in the RAID-Z group. To
double your read IOPS, you would need to halve the number of "data"
disks in the RAID-Z group (e.g. with RAIDZ-2, go from 12 to 7 disks).
Note that streaming read performance is independent of RAIDZ
configuration, because only the data is read. Streaming write
performance is proportional to space efficiency.

For space efficiency, typically doubling the number of "data" disks will
halve the amount of parity per MB of data (e.g. with RAIDZ-2, going from
7 to 12 disks will reduce the amount of parity information from 40% to
20%).

RAID-Z block layout RAID-Z parity information is associated with each
block, rather than with specific stripes as with RAID-4/5/6. Take for
example a 5-wide RAIDZ-1. A 3-sector block will use one sector of parity
plus 3 sectors of data (e.g. the yellow block at left in row 2). A
11-sector block will use 1 parity + 4 data + 1 parity + 4 data + 1
parity + 3 data (e.g. the blue block at left in rows 9-12). 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". 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. too small to fit even a single sector of
data plus p parity sectors - e.g. the light blue block at left in rows
8-9 with 1 parity + 2 data + 1 padding). Therefore, RAID-Z requires a
bit more space for parity and overhead than RAID-4/5/6.

A 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. 2
for RAIDZ-2), and n is an integer. These people would claim that for
example, a 9-wide (2^3+1) RAIDZ1 is better than 8-wide or 10-wide. This
is not generally true. The primary flaw with this recommendation is that
it assumes that you are using small blocks whose size is a power of 2.
While some workloads (e.g. databases) do use 4KB or 8KB logical block
sizes (i.e. recordsize=4K or 8K), these workloads benefit greatly from
compression. At Delphix, we store Oracle, MS SQL Server, and PostgreSQL
databases with LZ4 compression and typically see a 2-3x compression
ratio. This compression is more beneficial than any RAID-Z sizing. Due
to compression, the physical (allocated) block sizes are not powers of
two, they are odd sizes like 3.5KB or 6KB. This means that we can not
rely on any exact fit of (compressed) block size to the RAID-Z group
width.

To help understand where these (generally incorrect) recommendations
come from, and what the hypothetical benefit would be if you were to use
recordsize=8K and compression=off with various RAID-Z group widths, I
have created a spreadsheet which shows how much space is used for
parity+padding given various block sizes and RAID-Z group widths, for
RAIDZ1, 2, or 3. You can see that there are a few cases where, if
setting a small recordsize with 512b sector disks and not using
compression, using (2^n+p) disks uses substantially less space than one
less disk. However, more disks in the RAID-Z group is never worse for
space efficiency.

Note that setting a small recordsize with 4KB sector devices results in
universally poor space efficiency -- RAIDZ-p is no better than p-way
mirrors for recordsize=4K or 8K. The strongest valid recommendation
based on exact fitting of blocks into stripes is the following: If you
are using RAID-Z with 512-byte sector devices with recordsize=4K or 8K
and compression=off (but you probably want compression=lz4): use at
least 5 disks with RAIDZ1; use at least 6 disks with RAIDZ2; and use at
least 11 disks with RAIDZ3.

To summarize: Use RAID-Z. Not too wide. Enable compression.

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