linux-kernel - Re: Slow disks.

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Message-ID: <x49aajxsrh3.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 22 Dec 2010 11:27:20 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Rogier Wolff <R.E.Wolff@...Wizard.nl>
Cc:	Greg Freemyer <greg.freemyer@...il.com>,
	Bruno Prémont <bonbons@...ux-vserver.org>,
	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org
Subject: Re: Slow disks.

Rogier Wolff <R.E.Wolff@...Wizard.nl> writes:

> Unquoted text below is from either me or from my friend. 
>
>
> Someone suggested we try an older kernel as if kernel 2.6.32 would not
> have this problem. We do NOT think it suddenly started with a certain
> kernel version. I was just hoping to have you kernel-guys help with
> prodding the kernel into revealing which component was screwing things
> up....
[...]
> ata3.00: ATA-8: WDC WD10EARS-00Y5B1, 80.00A80, max UDMA/133

This is an "Advanced format" drive, which, in this case, means it
internally has a 4KB sector size and exports a 512byte logical sector
size.  If your partitions are misaligned, this can cause performance
problems.

> MDstat:
>
> Personalities : [raid1] [raid6] [raid5] [raid4]
> md125 : active raid5 sdd3[5](S) sdb3[4] sda3[0] sdc3[3]
>       3903488 blocks super 1.1 level 5, 512k chunk, algorithm 2 [3/3] [UUU]
>
> md126 : active raid5 sda4[0] sdd4[3] sdc4[5](S) sdb4[4]
>       1910063104 blocks super 1.1 level 5, 512k chunk, algorithm 2
> [3/3] [UUU]
>
> md1 : active raid5 sda2[0] sdd2[3](S) sdb2[1] sdc2[4]
>       39067648 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3]
> [3/3] [UUU]
>
> md1 : active raid5 sda2[0] sdd2[3](S) sdb2[1] sdc2[4]
>       39067648 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3]
> [UUU]

A 512KB raid5 chunk with 4KB I/Os?  That is a recipe for inefficiency.
Again, blktrace data would be helpful.

Cheers,
Jeff
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