Absolutely, I have done hot spot tests using a Poisson random
distribution. With that pattern (where there are many cache hits), the
writes are 3-10 times faster than sequential speed. My comment was
regarding purely random i/o across a large (at least much larger than
available memory cache) area. A real workload is likely to have a
combination of patterns, i.e. some fairly random, some hot spot, and
some sequential.
Chuck
-----Original Message-----
From: Roch Bourbonnais - Performance Engineering
[mailto:[EMAIL PROTECTED]
Sent: Thursday, May 11, 2006 1:18 AM
To: Gehr, Chuck R
Cc: [EMAIL PROTECTED]; Boyd Adamson; ZFS filesystem discussion
list
Subject: RE: [zfs-discuss] ZFS and databases
Gehr, Chuck R writes:
> One word of caution about random writes. From my experience, they
are > not nearly as fast as sequential writes (like 10 to 20 times
slower) > unless they are carefully aligned on the same boundary as the
file > system record size. Otherwise, there is a heavy read penalty
that you > can easily observe by doing a zpool iostat. So, depending
on the > workload, it's really a stretch to say random writes can be
done at > sequential speed.
>
> Chuck
>
Could we agree on saying that
partial writes to blocks that are not in cache are much
slower than writes to blocks that are.
Then given that Sequential pattern can benefit from
readahead, then those will fall in the fast category most of the time.
Performance of Random writes will depend on the
cached ratio. For DB working sets that greatly exceeds
system memory, which is common, then this fall in the slower case and
this stays true for any filesystem.
Or said otherwise, There is no free lunch.
-r
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