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

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Date:	Thu, 2 Apr 2009 21:14:00 -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

I'm really sorry, I just realized I hijacked this thread.  I'll stop now.

On Thu, Apr 2, 2009 at 9:13 PM, Trenton D. Adams
<trenton.d.adams@...il.com> wrote:
> On Thu, Apr 2, 2009 at 8:58 PM, David Rees <drees76@...il.com> wrote:
>> On Thu, Apr 2, 2009 at 7:28 PM, Trenton D. Adams
>>> That's the odd thing, I was setting them to 2 and 1.  I was just
>>> looking at the 2.6.29 code, and it should have made a difference.  I
>>> don't know what version of the kernel I was using at the time.  And,
>>> I'm not sure if I had the 1M fsync tests in place at the time either,
>>> to be sure about what I was testing.  It could be that I wasn't being
>>> very scientific about it at the time.  Thanks though, that setting
>>> makes a huge difference.
>>
>> Well, it depends on how much memory you have.  Keep in mind that those
>> are percentages - so if you have 2GB RAM, that's the same as setting
>> it to 40MB and 20MB respectively - both are a lot larger than the 1M
>> you were setting the dirty*bytes vm knobs to.
>>
>> I've got a problematic server with 8GB RAM.  Even if set both to 1,
>> that's 80MB and the crappy disks I have in it will often only write
>> 10-20MB/s or less due to the seekiness of the workload.  That means
>> delays of 5-10 seconds worst case which isn't fun.
>>
>> -Dave
>>
>
> Yeah, I just finished doing the calculation. :P  40M is what I'm
> seeing.  Yeah, that sounds like the same as my problem.  Even setting
> it to 10M dirty_bytes has a very serious latency problem.  I'm glad
> that option was added, because 1M works much better.  I'll have to
> change my shell script to dynamically tune on that.  Because under
> normal load, I want the 40M+ of queueing.  It's just when things get
> really heavy, and stuff starts getting flushed, that this problem
> starts happening.
>
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