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

[Eric Blake <eblake@redhat.com>]

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On 05/10/2016 09:41 AM, Alex Bligh wrote:
> 
> On 10 May 2016, at 16:29, Eric Blake <eblake@redhat.com> wrote:
> 
>> So the kernel is currently one of the clients that does NOT honor block
>> sizes, and as such, servers should be prepared for ANY size up to
>> UINT_MAX (other than DoS handling).
> 
> Interesting followup question:
> 
> If the kernel does not fragment TRIM requests at all (in the
> same way it fragments read and write requests), I suspect
> something bad may happen with TRIM requests over 2^31
> in size (particularly over 2^32 in size), as the length
> field in nbd only has 32 bits.
> 
> Whether it supports block size constraints or not, it is
> going to need to do *some* breaking up of requests.

Does anyone have an easy way to cause the kernel to request a trim
operation that large on a > 4G export?  I'm not familiar enough with
EXT4 operation to know what file system operations you can run to
ultimately indirectly create a file system trim operation that large.
But maybe there is something simpler - does the kernel let you use the
fallocate(2) syscall operation with FALLOC_FL_PUNCH_HOLE or
FALLOC_FL_ZERO_RANGE on an fd backed by an NBD device?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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