Erik Trimble wrote:
The main reason I don't see ZFS mirror / HW RAID5 as useful is this:
ZFS mirror/ RAID5: capacity = (N / 2) -1
speed << N / 2 -1
minimum # disks to lose before loss
of data: 4
maximum # disks to lose before loss
of data: (N / 2) + 2
ZFS mirror / HW Stripe capacity = (N / 2)
speed >= N / 2
minimum # disks to lose before loss
of data: 2
maximum # disks to lose before loss
of data: (N / 2) + 1
Given a reasonable number of hot-spares, I simply can't see the (very)
marginal increase in safety give by using HW RAID5 as out balancing the
considerable speed hit using RAID5 takes.
Eric,
Your analysis lacks some very important views of the problem.
0. Probability of failure is not constant across the components involved.
1. Disks don't tend to fail completely as often as they fail partially.
For partial failures, the recovery method is very different for the
various hardware RAID types and ZFS.
2. Analysis for data availability is different than analysis for data loss
and performance. Typically, we do a performability analysis which
shows the relationship between availability and performance. Data loss
analysis is handled differently, as it is often measured in years
(perhaps tens of thousands of years) and is highly dependent upon
maintenance activity.
3. For most hardware RAID arrays, RAID-5 performance is similar to RAID-1+0.
In order to assign a value to the performance envelope, something must
be known about the workload. RAID-6 or raidz2 performs ???
4. Scrubbing methods are also different between ZFS and RAID arrays. This
does impact latent fault detection which in turn impacts data loss.
Depending on requirements, we might recommend something fast, but risky, or
something designed to never forget. Saying that some configuration has
little value only applies to a specific set of requirements.
-- richard
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