On Sep 4, 2009, at 6:33 PM, Bob Friesenhahn <bfrie...@simple.dallas.tx.us
> wrote:
On Fri, 4 Sep 2009, Scott Meilicke wrote:
I only see the blocking while load testing, not during regular
usage, so I am not so worried. I will try the kernel settings to
see if that helps if/when I see the issue in production.
The flipside of the "pulsing" is that the deferred writes dimish
contention for precious read IOPs and quite a few programs have a
habit of updating/rewriting a file over and over again. If the file
is completely asynchronously rewritten once per second and zfs
writes a transaction group every 30 seconds, then 29 of those
updates avoided consuming write IOPs. Another benefit is that if
zfs has more data in hand to write, then it can do a much better job
of avoiding fragmentation, avoid unnecessary COW by diminishing
short tail writes, and achieve more optimum write patterns.
I guess one can find a silver lining in any grey cloud, but for myself
I'd just rather see a more linear approach to writes. Anyway I have
never seen any reads happen during these write flushes.
-Ross
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