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Re: [Qemu-devel] [Nbd] [PATCH] nbd: fix trim/discard commands with a len

From: Quentin Casasnovas
Subject: Re: [Qemu-devel] [Nbd] [PATCH] nbd: fix trim/discard commands with a length bigger than NBD_MAX_BUFFER_SIZE
Date: Tue, 10 May 2016 18:27:50 +0200
User-agent: Mutt/1.5.24 (2015-08-30)

On Tue, May 10, 2016 at 05:23:21PM +0100, Alex Bligh wrote:
> On 10 May 2016, at 17:04, Quentin Casasnovas <address@hidden> wrote:
> > Alright, I assumed by 'length of an NBD request', the specification was
> > talking about the length of.. well, the request as opposed to whatever is
> > in the length field of the request header.
> With NBD_CMD_TRIM the length in the header field is 32 bit and specifies
> the length of data to trim, not the length of the data transferred (which
> is none).
> > 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.
> > I'm just finding odd that something that fits inside the length field can't
> > be used.
> That's a different point. That's Qemu's 'Denial of Service Attack'
> prevention, *not* maximum block sizes. It isn't dropping it because
> of a maximum block size parameter. If it doesn't support the block size
> extension which the version you're looking at does not, it's meant
> to handle requests up to 2^32-1 long EXCEPT that it MAY error requests
> so long as to cause a denial of service attack. As this doesn't fit
> into that case (it's a TRIM), it shouldn't be erroring it on that grounds.
> I agree Qemu should fix that.
> (So in a sense Eric and I are arguing about something irrelevant to
> your current problem, which is how this would work /with/ the block
> size extensions, as Eric is in the process of implementing them).

Riight!  OK understood, thanks for the explanation.


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