Miles Nordin wrote:
For example, Postfix will do one or two fsync()'s per mail it receives
from the Interent into its internal queue, but I think it can be
working on receiving many mails at once, all waiting on fsync's
together. If the filesystem stack pushes these fsync's all the way
down to the disk and serializes them, you'll only end up with a few
parallel I/O transactions open, because almost everything postfix
wants to do is ``write, sync, let me know when done.''
OTOH, if you ran three instances of Postfix on the same single box
with three separate queue diectories on three separate
filesystems/zpools, inside zones for example, and split the mail load
among them using MX records or a load balancer, you might through this
ridiculous arrangement increase the parallelism of I/O reaching the
disk even on a system that serializes fsync's.
but if ZFS is smart enough to block several threads on fsync at once,
batch up their work to a single ZIL write-and-sync, then the
three-instance scheme will have no benefit.
ZFS does exactly this.
I demonstrate it on the SSD Discovery Days I run periodically in the UK.
--
Andrew Gabriel
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