Unbalanced reads of RAID10

[Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>]

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.

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?

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?

Thanks,

bye,

-- 

piergiorgio