I assembled a 3-component raid1 out of 3 4GB partitions.
After syncing, I ran the following script:
for bs in 32 64 128 192 256 384 512 768 1024 ; do \
let COUNT="2048 * 1024 / ${bs}"; \
echo -n "${bs}K bs - "; \
dd if=/dev/md1 of=/dev/null bs=${bs}k count=$COUNT iflag=direct 2>&1 |
grep 'copied' ; \
done
I also ran 'dstat' (like iostat) in another terminal. What I noticed was
very unexpected to me, so I re-ran it several times. I confirmed my
initial observation - every time a new dd process ran, *all* of the read
I/O for that process came from a single disk. It does not (appear to)
have to do with block size - if I stop and re-run the script the next
drive in line will take all of the I/O - it goes sda, sdc, sdb and back
to sda and so on.
I am getting 70-80MB/s read rates as reported via dstat, and 60-80MB/s
as reported by dd. What I don't understand is why just one disk is being
used here, instead of two or more. I tried different versions of
metadata, and using a bitmap makes no difference. I created the array
with (allowing for variations of bitmap and metadata version):
mdadm --create --level=1 --raid-devices=3 /dev/md1 /dev/sda3 /dev/sdb3 /dev/sdc3
I am running 2.6.18.8-0.3-default on x86_64, openSUSE 10.2.
Am I doing something wrong or is something weird going on?
--
Jon Nelson <jnelson-linux-raid@xxxxxxxxxxx>
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