Hello Neil and all,
first off thanks for your interest.
On Sat, 16 Aug 2008, Neil Brown wrote:
You should only need about 12Meg of dirty memory to keep your RAID0
busy, and I doubt that is even 1% of your memory.
Strange. With the 12disk raid we get as much as 200MB dirty and throughput
is slow. With the 4*3disk raid's we see just around 12MB and throughput
is good. We have 4GB RAM.
md/raid0 has very little overhead. It just encourages the filesystem
to send requests that are aligned with the chunks, and then sends each
request on to the target drive without any further intervention. So
the queue for each device should be kept full, and the individual
devices should be going at full speed.
Okay. We have been using power-of-2 raw write sizes, and in the case of
using a filesystem, it was XFS with settings in mkfs.xfs that tell it
about the chunk size and device count.
So to make sure I'm not misunderstanding you description, could you
run the two tests again: 12disk raid0 and 4*3disk raid0, and for
each report
mdadm -D of each array
output of
time dd if=/dev/zero of=/dev/mdXX bs=1024k
on all arrays in parallel
Using a chunksize of 1024.
One of our eSATA<=>SATA conversion cables got lost, so I couldn't right
now do your test on the 4*3disk raid0 in my first posting. Instead I ran
with 12disk and 3*4disk raid0.
http://www.metsahovi.fi/~jwagner/10g-tests/pmp/linux-raid-1.txt
473 MB/s with single raid, high 'dirty', 692 MB/s with triple raid and low
'dirty.
Any ideas?
TIA,
- Jan
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