linux-kernel - Re: Queuing of disk writes

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Date:	Fri, 1 Apr 2011 13:34:24 -0700
From:	Charles Samuels <charles@...iden.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Queuing of disk writes

On Friday, April 01, 2011 1:10:49 pm Alan Cox wrote:
> > the kernel's write cache, and then consequently the disk drive's DMA
> > queue. As a result of that, the harddrive can pick the correct order to
> > do these writes, significantly reducing seek times.
> 
> Well that depends a lot on the data, if its very scattered and random it
> may not help much.
Generally 64KiB - I don't know disk geometry, but I guess that's a lot smaller 
than a cylinder.

But I don't really care about throughput (much), I care more about my 
application not blocking everything while the fsync happens.

> 
> > And yes, I *know* fsync is a poor method to determine if data is actually
> > committed to something non-volatile. :)
> 
> fsync/fdatasync should at least make sure it hit the disk. If barriers
> are enabled the rest too.
> 
> What file system are you using - some of the file systems have serious
> limits in this are around fsync and ordering and you may be hitting those.

I've seen this on ext3, ext4, and XFS. Reiser3 not so much. I've also 
convinced my users to use write-backed cache. They're Enterprise customers and 
have loads of money to spend on hardware, such as Warp Cores.

> 
> The ultima answer is probably an SSD of course 8)
Well, that would solve the throughput, but it doesn't solve the blocking!

Charles
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