linux-kernel - Re: [PATCH 0/2] block: align start sector of a discard request

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Message-ID: <4F71617F.60103@redhat.com>
Date:	Tue, 27 Mar 2012 08:43:11 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Christoph Hellwig <hch@....de>
CC:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 0/2] block: align start sector of a discard request

Il 14/03/2012 18:12, Paolo Bonzini ha scritto:
> When a disk has a large discard_granularity, discards are not split with
> optimal alignment; the pessimization gets bigger as discard_granularity
> and max_discard_sectors become closer.
> 
> Take the limit case of discard_granularity == max_discard_sectors == 64.
> Then, if a request is submitted for 256 sectors 2..257 it will be split
> like this: 2..65, 66..129, 130..193, 194..257.  None of these requests
> is aligned, so in fact you might end up with no discarded logical blocks
> at all.  With this patch, the split will be 2..63, 64..127, 128..191,
> 192..255, 256..257.  The patches also take the discard_alignment into
> consideration.
> 
> Patch 1 adjusts the computation of the granularity-adjusted
> max_discard_sectors so that it prepares for the new code in patch 2,
> while patch 2 actually adjusts the split.
> 
> Paolo Bonzini (2):
>   block: tweak rounding of max_discard_sectors
>   block: split discard into aligned requests
> 
>  block/blk-lib.c    |   41 +++++++++++++++++++++++++++--------------
>  1 files changed, 27 insertions(+), 14 deletions(-)
> 

Christoph, since you talked about this on the qemu-devel mailing list,
do you have a few free cycles to review this?

Paolo
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