ozan s. yigit wrote:
for s10u2, documentation recommends 3 to 9 devices in raidz. what is the
basis for this recommendation? i assume it is performance and not failure
resilience, but i am just guessing... [i know, recommendation was intended
for people who know their raid cold, so it needed no further explanation]
Both actually.
The small, random read performance will approximate that of a single disk.
The probability of data loss increases as you add disks to a RAID-5/6/Z/Z2
volumes.
For example, suppose you have 12 disks and insist on RAID-Z.
Given
1. small, random read iops for a single disk is 141 (eg. 2.5" SAS
10k rpm drive)
2. MTBF = 1.4M hours (0.63% AFR) (so says the disk vendor)
3. no spares
4. service time = 24 hours, resync rate 100 GBytes/hr, 50% space
utilization
5. infinite service life
Scenario 1: 12-way RAID-Z
performance = 141 iops
MTTDL[1] = 68,530 years
space = 11 * disk size
Scenario 2: 2x 6-way RAID-Z+0
performance = 282 iops
MTTDL[1] = 150,767 years
space = 10 * disk size
[1] Using MTTDL = MTBF^2 / (N * (N-1) * MTTR)
-- richard
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