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

From: Paolo Bonzini
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 19:55:56 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 10/05/2016 17:38, Alex Bligh wrote:
> > and are at the
> > mercy of however the kernel currently decides to split large requests).
> I am surprised TRIM doesn't get broken up the same way READ and WRITE
> do.

The payload of TRIM has constant size, so it makes sense to do that.
The kernel then self-imposes a 2GB limit in blkdev_issue_discard.

On the other hand, READ and WRITE of size n can possibly require O(n)


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