Re: Unbalanced reads of RAID10

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Wed, Sep 02, 2009 at 12:13:15PM -0400, Bill Davidsen wrote:
> Keld Jørn Simonsen wrote:
>> On Sun, Aug 23, 2009 at 04:24:39PM +0200, Piergiorgio Sartor wrote:
>>   
>>> Hi all,
>>>
>>> some time ago I was reporting about a strange issue.
>>>
>>> I have a two HDDs system, with a small RAID1 (/boot)
>>> and the rest as RAID10 f2 (with LVM on top).
>>>
>>> It seems that /dev/sdb has more reads than /dev/sda.
>>>
>>> I had a quick check, with "iostat", and it seems that
>>> all small reads, somehow below 1~4KiB, are done from
>>> /dev/sdb2, regardless.
>>> Actually, it seems that only if there is a pending
>>> (small) read, this will be scheduled to /dev/sda2,
>>> but non-overlapping small reads seem to happen always
>>> from /dev/sdb2.
>>>
>>> This occurs with the RAID10, but it seems also with
>>> the RAID1.
>>>     
>>
>> Hmm, have you done testing separately on each array?
>>
>>   
>>> In normal operation, this does not seem to lead to
>>> problems, but during the smart long test /dev/sdb
>>> takes by far more time than /dev/sda, since each
>>> small read stop the test, and small read occurs
>>> whenever there is a small write from syslog or
>>> similar.
>>> Note that failing and removing /dev/sdb2 results
>>> in much shorter time for the smart test, about
>>> 1hr30min vs. the 6~7hrs with the drive still
>>> attached to RAID10.
>>>
>>> Is there any way to tune which is the "preferred"
>>> drive or the "preferred" policy in case of these
>>> small (or big) reads?
>>>     
>>
>> What level of the kernel are you running?
>>
>>   
>>> Could this be due to HW configuration?
>>> The two HDDs are numbered SATA1 and SATA2 in BIOS,
>>> there are still SATA3 and SATA4 ports somehow
>>> available (SATA3 has a DVD).
>>>
>>> How are the reads scheduled withing the RAID10 software?
>>>     
>>
>> there was a change of this about 2.6.25 which forced reads to always be
>> from the faster inner part of the disks, and that should even out reads.
>>   
>
> ??? what faster inner part is that? The linear velocity and  
> bytes/cylinder are higher as the diameter increases.

I meant the faster lower numbered sectors, which are the *outer*
sectors on the physical disk.

Best regards
Keld
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