Re: [Qemu-trivial] [Nbd] [Qemu-devel] [PATCH] nbd: fix trim/discard commands with a length bigger than NBD_MAX_BUFFER_SIZE

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/05/2016 18:23, Alex Bligh wrote:
> > Is there a use case where you'd want to split up a single big TRIM request
> > in smaller ones (as in some hardware would not support it or something)?
> > Even then, it looks like this splitting up would be hardware dependant and
> > better implemented in block device drivers.
> 
> Part of the point of the block size extension is to push such limits to the
> client.
> 
> I could make up use cases (e.g. that performing a multi-gigabyte trim in
> a single threaded server will effectively block all other I/O), but the
> main reason in my book is orthogonality, and the fact the client needs
> to do some breaking up anyway.

That's why SCSI for example has a limit to UNMAP and WRITE SAME requests
(actually it has three limits: number of ranges per unmap, which in NBD
and in SCSI WRITE SAME is 1; number of blocks per unmap descriptor;
number of blocks per WRITE SAME operation).  These limits however are a
different one than the read/write limit, because you want to support
systems where TRIM is much faster than regular I/O (and possibly even
O(1) if trimming something that is already trimmed).

Paolo