On 16-08-12 1:05 PM, Stan Hoeppner wrote:
On 8/15/2012 6:07 PM, Miquel van Smoorenburg wrote:
Ehrm no. If you modify, say, a 4K block on a RAID5 array, you just have
to read that 4K block, and the corresponding 4K block on the
parity drive, recalculate parity, and write back 4K of data and 4K
of parity. (read|read) modify (write|write). You do not have to
do I/O in chunksize, ehm, chunks, and you do not have to rmw all disks.
See: http://www.spinics.net/lists/xfs/msg12627.html
Dave usually knows what he's talking about, and I didn't see Neil nor
anyone else correcting him on his description of md RMW behavior.
Well he's wrong, or you're interpreting it incorrectly.
I did a simple test:
* created a 1G partition on 3 seperate disks
* created a md raid5 array with 512K chunksize:
mdadm -C /dev/md0 -l 5 -c $((1024*512)) -n 3 /dev/sdb1 /dev/sdc1
/dev/sdd1
* ran disk monitoring using 'iostat -k 5 /dev/sdb1 /dev/sdc1 /dev/sdd1'
* wrote a single 4K block:
dd if=/dev/zero bs=4K count=1 oflag=direct seek=30 of=/dev/md0
Output from iostat over the period in which the 4K write was done. Look
at kB read and kB written:
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb1 0.60 0.00 1.60 0 8
sdc1 0.60 0.80 0.80 4 4
sdd1 0.60 0.00 1.60 0 8
As you can see, a single 4K read, and a few writes. You see a few blocks
more written that you'd expect because the superblock is updated too.
Mike.
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