Oops, I was off by a factor of 1000 in my original subject. We actually have 4M and 8M reads being split into 100 512kB reads per second. So perhaps these are limiting:
# cat /sys/block/vdb/queue/max_sectors_kb
512
# cat /sys/block/vdb/queue/read_ahead_kb
512
Questions below remain.
Cheers, Dan
On 27 Nov 2014 18:26, Dan Van Der Ster <daniel.vanderster@xxxxxxx> wrote:
Hi all,
We throttle (with qemu-kvm) rbd devices to 100 w/s and 100 r/s (and 80MB/s write and read).
With fio we cannot exceed 51.2MB/s sequential or random reads, no matter the reading block size. (But with large writes we can achieve 80MB/s).
I just realised that the VM subsytem is probably splitting large reads into 512 byte reads, following at least one of:
# cat /sys/block/vdb/queue/hw_sector_size
512
# cat /sys/block/vdb/queue/minimum_io_size
512
# cat /sys/block/vdb/queue/optimal_io_size
0
vdb is an RBD device coming over librbd, with rbd cache=true and mounted like this:
/dev/vdb on /vicepa type xfs (rw)
Did anyone observe this before?
Is there a kernel setting to stop splitting reads like that? or a way to change the io_sizes reported by RBD to the kernel).
(I found a similar thread on the lvm mailing list, but lvm shouldn’t be involved here.)
All components here are running latest dumpling. Client VM is running CentOS 6.6.
Cheers, Dan
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