I have not yet had a chance to play with either XSan 2 or with the
Promise RAIDs. I'm wondering how much of the hard-won knowledge from
XSan 1 and the XRAID is still applicable. Specifically, I wonder if
these are still true:
* Under XSan 1, you were expected to dedicate an entire RAID
controller to the metadata partition. This meant that for small
(single XRAID) configurations, you immediately lost a major portion of
the available storage. On a single XRAID setup, you lost 50% of the
notional storage capacity.
* Under XSan 1, we observed a major performance hit for each RAID
controller. Keeping in mind that those were the old 200MB/sec ports
rather than the fancy new 400MB/sec ones ... I observed data transfers
on the order of 150 - 180MB/sec with a single controller directly
attached to a Mac Pro, and up to 360MB/sec striping a partition across
a pair of them. Under XSan we observed up to 100MB/sec, per
controller involved in a storage pool. The throughput of a storage
pool was (generally speaking) the sum of the throughputs of the
controllers. This meant (at least for me, in practice) that smaller
installations saw significantly lower performance than they could have
achieved using a directly attached RAID and NFS or AFP. The turning
point on performance was three XRAID chassis.
What is the current wisdom on small XSan deployments?
-Chris Dwan
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