Re: Linux disk performance.
From: Nick Piggin
Date: Mon Dec 18 2006 - 08:09:57 EST
Manish Regmi wrote:
On 12/18/06, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:
if you want truely really smooth writes you'll have to work for it,
since "bumpy" writes tend to be better for performance so naturally the
kernel will favor those.
to get smooth writes you'll need to do a threaded setup where you do an
msync/fdatasync/sync_file_range on a frequent-but-regular interval from
a thread. Be aware that this is quite likely to give you lower maximum
performance than the batching behavior though.
Thanks...
But isn't O_DIRECT supposed to bypass buffering in Kernel?
Doesn't it directly write to disk?
I tried to put fdatasync() at regular intervals but there was no
visible effect.
I don't know exactly how to interpret the numbers you gave, but
they look like they might be a (HZ quantised) delay coming from
block layer plugging.
O_DIRECT bypasses caching, but not (all) buffering.
Not sure whether the block layer can handle an unplug_delay set
to 0, but that might be something to try (see block/ll_rw_blk.c).
--
SUSE Labs, Novell Inc.
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