Re: Linux MD RAID 1 read performance tunning

["Ciprian Dorin, Craciun" <ciprian.craciun@gmail.com>]

2009/12/22 Keld Jørn Simonsen <keld@keldix.com>:
> On Tue, Dec 22, 2009 at 06:34:55PM +0200, Ciprian Dorin, Craciun wrote:
>>     Hello all!
>>
>>     I've created a 64G RAID 1 matrix from 3 real disks. (I intend to
>> use this as a target for backups.)
>>     Now while playing around with this array, I've observed that the
>> read performance is quite low because it always reads from the disk in
>> the first slot (which happens to be the slowest...)
>>
>>     So my questions are:
>>     * is there any way to tell the MD driver to load-balance the reads
>> between the three disks?
>
> It does not make sense to do distributed reading in raid1 for sequential
> files. This is because it will not be faster to read from more drives,
> as this will only make the reading from one drive skipping blocks on
> that drive. In other words, in the time you use for skipping blocks on
> one drive, you could just as well have read the blocks. So then better
> just read all the blocks off one drive, and then do other possible IO
> from other drives.

    Aha. It makes sens now. But, does it mean that if I have parallel
IO's (from different read operations) they are going to be distributed
between the disks?


> RAID10 is a modern form of raid1, which can do better distributed
> reading, especially raid10,f2 is good for disks, while raid10,o2 may be
> better for SSDs.
>
> Best regards
> Keld

    Unfortunately my setup forces me to use RAID1. (Because any of the
three drives should be usable for a full recovery of the backup data.)
(P.S.: Just to be clear, I'm not using RAID as a backup solution, but
rather a redundancy solution for the backup solution. :) )

    Thanks.
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