raid1 performance

[Jaime Medrano]

I have several raid arrays (level 0 and 1) in my machine and I have
noticed that raid1 is much more slower than I expected.

The arrays are made from two equal hds (/dev/hde, /dev/hdg). And some
numbers about the read performances are:

/dev/hde: 29 Mb/s
/dev/hdg: 29 Mb/s
/dev/md0: 27 Mb/s (raid1)
/dev/md1: 56 Mb/s (raid0)
/dev/md2: 27 Mb/s (raid1)

These numbers comes from hdparm -tT. I have noticed a very poor
performance when reading sequentially a large file from raid1 (I suppose
this is what hdparm does).

I have taken a look at the read balancing code at raid1.c and I have found
that when a sequential read happens no balancing is done, and so all the
reading is done from only one of the mirrors while the others are iddle.ç

I have tried to modify the balancing algorithm in order to balance also
sequential access, but I have got almost the same numbers.

I have thought that the reason may be that some layer bellow is making
reads of greater size than the chunks in which I balance, and so the same
work is being done twice; but I don't know the way to find this.

Does anybody know how this works?

Regards,
Jaime Medrano