Re: Choice of BDRV_REQUEST_MAX_SECTORS

[Eric Blake]

On 8/17/20 7:32 AM, Peter Lieven wrote:
> Hi,
>
>
> I am currently debugging a performance issue in qemu-img convert. I 
> think I have found the cause and will send a patch later.
>
> But is there any reason why BDRV_REQUEST_MAX_SECTORS is not at least 
> aligned down to 8 (4k sectors)?
>
> Any operation that is not able to determinate an optimal or maximum 
> request size from a driver will likely use this value and
>
> might generate unaligned requests (as qemu-img convert does) as soon as 
> the first iteration of an operation is done.

Vladimir's work to make the block layer 64-bit clean should help resolve 
this, as we will be relying more heavily on values aligned to the 
driver's limits rather than arbitrary 32-bit cutoffs.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org