Re: large reads become 512 kbyte reads on qemu-kvm rbd

[Dan Van Der Ster <daniel.vanderster@xxxxxxx>]

Hi,

After some more tests we’ve found that max_sectors_kb is the reason for splitting large IOs.

We increased it to 4MB:

   echo 4096 > cat /sys/block/vdb/queue/max_sectors_kb

and now fio/iostat are showing reads up to 4MB are getting through to the block device unsplit.

We use 4MB to match the size of the underlying RBD objects. I can’t think of a reason to split IOs smaller than the RBD objects -- with a small max_sectors_kb the client would use 8 IOs to read a single object.

Does anyone know of a reason that max_sectors_kb should not be set to the RBD object size? Is there any udev rule or similar that could set max_sectors_kb when a RBD device is attached?

Cheers, Dan

On 27 Nov 2014, at 20:29, Dan Van Der Ster <daniel.vanderster@xxxxxxx> wrote:

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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