pauln wrote:
On our ddn's I've noticed that during large reads the backend
bandwidth of the ddn is pegged (700-800 MB/s) while the total
bandwidth being delivered through the fc interfaces is in the low
hundreds of MBs. This leads me to believe that the ddn cache is being
thrashed heavily due to overly aggressive read-ahead by many OSTs and
the ddn itself.
I haven't re-run these tests with the 2.6 kernel (cray 1.4XX) so I'm
not sure if this phenomena still exists but this theory still may have
some validity since the ddn cache is shared between all the osts
connected to it.
paul
Paul -- what is the "prefetch" value for the LUNs? The "cache" command
is where this can be found.
Historical testing on s2a8500s shows that for Lustre, prefetch=1 or
prefetch=0 is desired. When run with prefetch=8, we saw the same
behavior you are describing, namely the DDN "stomping" on itself trying
to prefetch too much data.
This testing has been done twice on the XT3, both to the same result --
prefetch=1.
Nic
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