The usual rule of thumb quoted is 80-100MB/s. This is what I was told
in the Xsan classes that I took and it also mentioned in the "Xsan
Quick-Reference Guide". This is what I have seen in my own use.
Now obviously, the 380MB/s (190MB/s per controller) on a direct
attached RAID has to be the most optimistic scenario. I'm not privy
to whatever tests were done, but that's 95% of the nominal 200MB/s
capacity of a FC cable. Figuring a seven disk raid 5 that works out
to ~31MB/s per disk which is about right. (Pull out a drive and look
up the specs.)
Where's half the performance go? I wish I knew. Even inside Apple one
is not allowed to know how the Xsan filesystem is put together.
-eryk
On Feb 13, 2006, at 7:51 AM, Christopher Dwan wrote:
I've been working with a couple of smaller XRaid systems with and
without XSan, and my empirical observation is that introducing XSan
slows down data access quite a bit, at least in our use cases. I'm
guess I'm wondering about the conflict between this piece of
marketing from apple.com/xserve/raid/performance.html:
Finely tuned, Independent Motors
The advanced Xserve RAID architecture delivers fast access to
storage without compromising data integrity. Dual independent RAID
controllers provide protected storage with unprecedented
performance. In fact, Xserve RAID boasts a throughput of over 380MB/
s, fast enough to support real-time, uncompressed, high-definition
(HD) and multiple-stream (SD) video editing without dropping a frame:
And this recent post:
The actual throughput on the FC controller on the XServe RAID is
somewhere around 80MB/s?
Is it XSan, or some other factor, that introduces the 80MB/s
limitation per controller on the XRaid?
My observation is that read / writes are considerably faster on a
directly mounted XRaid than on one made available through XSan, all
else being basically equal. Obviously, XSan scales where direct
mounts do not, but is there really a factor of >2 performance hit
on for very small SANs?
-Chris Dwan
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