Robert Milkowski wrote:
Hello Richard,
Friday, October 13, 2006, 8:05:18 AM, you wrote:
REP> Do you want data availability, data retention, space, or performance?
data availability, space, performance
Thumper is not designed for high data availability, more traditional RAID
arrays are fully redundant. Thumper is designed for long data retention,
which is a slightly different design point. Thumper's controllers and
processors can be replaced without affecting the data, but the data will
be inaccessible during such procedures.
However we're talking about quite a lot of small IOs (r+w).
The real question was what do you think about creating each raid group
only from disks from different controllers so controller failure won't
affect data availability.
I have modeled this and have the predicted reliability data. In a nutshell,
don't worry about the controllers. As Matt pointed out, each disk has a
direct, unshared connection to the controller and each controller is on its
own PCI bus. These are highly reliable components, much more reliable than
the disks, power supplies, DIMMs, and fans.
The choice of data redundancy is much more important for achieving a level
of happiness. However, for lots of small iops, RAID-Z or RAID-Z2 will be
a potential dissatisfier unless you use a large number of sets. To the
first order, you can consider the performance of a RAID-Z or RAID-Z2 set
for small, random reads to be equivalent to the performance of a single
disk. Writes won't be so bad under ZFS, but we don't have a good performance
model for small, random writes yet.
-- richard
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