quoted

Hi Roman,
thank you for your hint, I verified the read-ahead settings and they are

the

quoted

same for all the block devices involved in the test: the value is 256

for all

quoted

/dev/sd?? and for all /dev/md?
there should be something else which is influencing  raid 1 performance.
Have someone of you ever had a similar issue ?

Very odd.
I just tested my test hardware and get exactly the same 56 MB/sec both for
the RAID1 and the individual devices.

There is only one way that I can think of that the accesses going via
RAID1
would be different from those going direct, and that is that the starting
offset might be different if you are using 1.x metadata.
I guess if you had those new 4K-sector devices that might make a
difference,
but I wouldn't really expect it to.

For a sequential read like that, md/raid1 doesn't even do read-balancing,
all
the reads go the the same device.

If you look at /proc/diskstats and particularly the 4th and 6th fields for
the device that you are interested in, and then take the differences for
each
field between 'before' and 'after' running a test you will get
  - the number of IO requests
  - the number of sectors

that were serviced during that time.  Taking a ratio will get you the
number
of sectors per IO.  Normally more is better.

I just tested 'sde' which gave 8.25 sectors per request - so most requests
were 4K.
md2 on the other hand gave 31.02, so many requests were 16K.  That really
surprises me.
Looking at he 'queue' numbers in /sys/block/X/queue - some of which guide
the
breaking up of pages into requests - all the md2 number are the same as
sde
or smaller.  So I'm currently rather confused.

It might be interesting to find out what the data offset is for your RAID1
(mdadm --examine will tell you if there is one), and compare the
request/sector numbers and see if they show anything.

	Interesting.  My video server has been up for 11 days since my last
reboot, and the individual members are showing an average of 173
sectors/request, while the RAID6 array is showing 167.  Meanwhile, the
backup server (up 9 days) is showing an average of 309 sectors/request,
while the RAID6 array there is showing 210.  Are these high numbers compared
to yours because most of the accesses are sequential?