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Re: [Qemu-trivial] [Nbd] [Qemu-devel] [PATCH] nbd: fix trim/discard comm

From: Eric Blake
Subject: Re: [Qemu-trivial] [Nbd] [Qemu-devel] [PATCH] nbd: fix trim/discard commands with a length bigger than NBD_MAX_BUFFER_SIZE
Date: Wed, 11 May 2016 08:08:43 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 05/11/2016 03:38 AM, Paolo Bonzini wrote:
> 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).

Then I think I will propose a doc patch to the extension-info branch
that adds new INFO items for NBD_INFO_TRIM_SIZE and
NBD_INFO_WRITE_ZERO_SIZE (if requested by client and replied by server,
then this can be larger than the normal maximum size in
NBD_INFO_BLOCK_SIZE; if either side doesn't request the info, then
assume any maximum in NBD_INFO_BLOCK_SIZE applies, otherwise UINT32_MAX;
and the two infos are separate items because NBD_FLAG_SEND_TRIM and
NBD_FLAG_SEND_WRITE_ZEROES are orthogonally optional).

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

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