Neil Brown wrote:
On Tuesday October 17, mirek@xxxxxxxxxxxxxxxx wrote:
I would like to propose an enhancement of raid 1 driver in linux kernel.
The enhancement would be speedup of data reading on mirrored partitions.
The idea is easy.
If we have mirrored partition over 2 disks, and these disk are in sync, there is
possibility of simultaneous reading of the data from both disks on the same way
as in raid 0. So it would be chunk1 read from master, chunk2 read from slave at
the same time.
As result it would give significant speedup of read operation (comparable with
speed of raid 0 disks).
This is not as easy as it sounds.
Skipping over blocks within a track is no faster than reading blocks
in the track, so you would need to make sure that your chunk size is
larger than one track - probably it would need to be several tracks.
What you said is certainly true when we read one file at a given moment.
What if we read two different files at a given time? Certainly, it would
be faster if DRIVE_1 reads FILE_1, and DRIVE_2 reads FILE_2.
Raid1 already does some read-balancing, though it is possible (even
likely) that it doesn't balance very effectively. Working out how
best to do the balancing in general in a non-trivial task, but would
be worth spending time on.
Probably what I said before isn't very correct, as RAID-1 has no idea of
the filesystem that is on top of it; rather, it will see attempts to
access differend areas of the array?
--
Tomasz Chmielewski
http://wpkg.org
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