Hi,
I have been doing some testing with striped rbd images and have a
question about the calculation of the optimal_io_size and
minimum_io_size parameters. My test image was created using a 4M object
size, stripe unit 64k and stripe count 16.
In the kernel rbd_init_disk() code:
unsigned int objset_bytes =
rbd_dev->layout.object_size * rbd_dev->layout.stripe_count;
blk_queue_io_min(q, objset_bytes);
blk_queue_io_opt(q, objset_bytes);
Which resulted in 64M minimal / optimal io sizes. If I understand the
meaning correctly then even for a small write there is going to be at
least 64M data written?
My use case is a ceph cluster (13.2.4) hosting rbd images for VMs
running on Xen. The rbd volumes are mapped to dom0 and then passed
through to the guest using standard blkback/blkfront drivers.
I am doing a bit of testing with different stripe unit sizes but keeping
object size * count = 4M. Does anyone have any experience finding
optimal rbd parameters for this scenario?
Thanks,
James
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