linux-kernel - Re: EXT4-ish "fixes" in UBIFS

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Date:	Thu, 2 Apr 2009 20:26:01 -0600
From:	"Trenton D. Adams" <trenton.d.adams@...il.com>
To:	David Rees <drees76@...il.com>
Cc:	Christian Kujau <lists@...dbynature.de>,
	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: EXT4-ish "fixes" in UBIFS

On Thu, Apr 2, 2009 at 7:55 PM, David Rees <drees76@...il.com> wrote:
> On Thu, Apr 2, 2009 at 5:28 PM, Trenton D. Adams
> <trenton.d.adams@...il.com> wrote:
>> On Thu, Apr 2, 2009 at 6:24 PM, Trenton D. Adams
>> <trenton.d.adams@...il.com> wrote:
>>> Yes, mounting "-o sync" does improve ext3 performance.  It sucks
>>> though, because I do want quick writes.  And mounting with sync option
>>> slows down to disk io speeds.  In my case, that's between 20 and 23
>>> megabytes per second *big frown, quivering lip, and tears in my eyes*.
>>> :P
>>>
>>
>> Oh, I should have clarified.  It improves performance under heavy
>> load.  Under normal load, mounting without sync is fine.  What I tend
>> to do is mount with "remount,rw,sync" when heavy load is starting.
>> Then my system goes slowly, but latency is good.  Then, when it's all
>> done (say a big compile, or job, or whatever), I remount without sync
>> again.
>>
>> I'm thinking of writing a script that monitors performance, and
>> remounts as needed, lol.  WHAT A HACK. hehe.
>
> All you're doing here is implementing the lowering of dirty data
> limits in the VM dynamically based on how long fsyncs take.
>
> Linus outlined this specific strategy as "the ideal siutation"
> somewhere in the depths of "That filesystem thread".
>
> Look at the new in 2.6.29 dirty*bytes parameters in
> Documentation/sysctl/vm.txt for more info.  By lowering those values,
> you can effectively turn normal writes into synchronous writes which
> will greatly reduce latency of fsync under heavy write load.

WOW, that makes a huge difference.  If I set it to 100M, I get the
10-15 second delay I was talking about.  But, if I set it to 1M, I get
0.3 to 0.4 second delay on a 1M fsync.  That is way better.  Perhaps I
should auto-tune based on that parameter then.  Although I do agree
with Linus that it sucks to do userland auto-tuning. :P
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