Re: Kernel driver I/O block size hinting
On Wed, Jun 15, 2022 at 11:09:31AM +0100, Richard W.M. Jones wrote:
> This kernel setting limits the maximum request size on the queue.
>
> In my testing reading and writing files with the default [above] the
> kernel never got anywhere near sending multi-megabyte requests. In
> fact the largest request it sent was 128K, even when I did stuff like:
>
> # dd if=/dev/zero of=/tmp/mnt/zero bs=100M count=10
>
> 128K happens to be 2 x blk_queue_io_opt, but I need to do more testing
> to see if that relationship always holds.
The answer is apparently no. With minimum_io_size == 64K and
optimal_io_size == 256K, the server still only sees at most 128K
requests.
Although I still think we need to make these changes to nbd.ko, I
don't think this is going to solve the original problem of trying to
aggregate requests into the very large block sizes favoured by S3.
(nbdkit blocksize filter + a layer of caching seems like the way to go
for that)
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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