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Date: Tue, 29 Oct 2013 00:48:32 +0400
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Allan Jude <freebsd@allanjude.com>
Subject: Re: ZFS txg implementation flaw
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On Mon, Oct 28, 2013 at 02:28:04PM -0400, Allan Jude wrote:

> On 2013-10-28 14:16, Slawa Olhovchenkov wrote:
> > On Mon, Oct 28, 2013 at 10:45:02AM -0700, aurfalien wrote:
> >
> >> On Oct 28, 2013, at 2:28 AM, Slawa Olhovchenkov wrote:
> >>
> >>> I can be wrong.
> >>> As I see ZFS cretate seperate thread for earch txg writing.
> >>> Also for writing to L2ARC.
> >>> As result -- up to several thousands threads created and destoyed per
> >>> second. And hundreds thousands page allocations, zeroing, maping
> >>> unmaping and freeing per seconds. Very high overhead.
> >>>
> >>> In systat -vmstat I see totfr up to 600000, prcfr up to 200000.
> >>>
> >>> Estimated overhead -- 30% of system time.
> >>>
> >>> Can anybody implement thread and page pool for txg?
> >> Would lowering vfs.zfs.txg.timeout be a way to tame or mitigate this?
> > vfs.zfs.txg.timeout: 5
> >
> > Only x5 lowering (less in real case with burst writing). And more fragmentation on writing and etc.
> > _______________________________________________
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> >From my understanding, increasing the timeout so you are doing fewer
> transaction groups, would actually be the way to increase performance,
> at the cost of 'bursty' writing and the associated uneven latency.

This (increasing the timeout) is dramaticaly decreasing read
performance by very high IO burst.