Re: Linux MD RAID 1 read performance tunning

[Goswin von Brederlow <goswin-v-b@web.de>]

Keld Jørn Simonsen <keld@keldix.com> writes:

> On Tue, Dec 22, 2009 at 07:08:25PM +0200, Ciprian Dorin, Craciun wrote:
>> 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?
>
> It should, but I am not fully sure it does.
> But try it out with two concurrent reads of two big files, and then
> watch it with iostat 
>
> Best regards
> keld

Actualy the kernel remembers the last read/write position for each
raid1 component and then uses the one which is nearest.

And when you read at 2/3 different positions at the same time then it
will use different components for each an use the same ones for
subsequent reads (as they will be nearer).

Try

dd if=/dev/md0 of=/dev/null bs=1M cunt=1024 skip=0 &
dd if=/dev/md0 of=/dev/null bs=1M cunt=1024 skip=1024 &
dd if=/dev/md0 of=/dev/null bs=1M cunt=1024 skip=2048

They should more or less get the same speed as a single dd.

MfG
        Goswin
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