linux-kernel - Re: Slow disks.

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Date:	Sun, 26 Dec 2010 15:38:51 -0800
From:	Mark Knecht <markknecht@...il.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Rogier Wolff <R.E.Wolff@...wizard.nl>,
	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.

On Wed, Dec 22, 2010 at 8:27 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> 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

I am late to this thread but I'd like to give a +1 to everything Jeff said.

I tried to build many different mdadm RAID configurations using the
WD10EARS. They simply didn't work. There were huge delays and mdadm
would take drives off line. It was a mess.

First, the drives are 4K sector so it's really important to get them
on the right partition boundary. Installing Gentoo requires that we
un-tar some large-ish files. IIRC on a 512 byte boundary these drives
took 20-30 minutes to do this. On a 4K boundary they took about 1
minute. I know that doesn't make sense but those were my numbers, and
it's a very high end Intel i7-980x MB so there wasn't any other
problem I could find.

Even when I got that part worked out I found that the drives just
didn't work well in a RAID. The mdadm list led me to believe that the
root cause was the lack of TLER in the firmware. I don't know how to
show that's true or not...

When all that was determined I dropped RAID and I still ran into Load
Count issue that's discussed elsewhere in this thread.

I then bought 5 WD RAID Edition drives and everything works perfectly.
> 100MB/Sec on RAID1 and about 180MB/S on RAID0.

Overall the WD10EARS drives were very disappointing, at least as
shipped. I have 5 of them sitting here unused. I would like to find
some time to try them again one of these days but I don't have high
hopes.

Cheers,
Mark
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