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Re: [Qemu-devel] [PATCHv3] block: introduce BDRV_O_SEQUENTIAL
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCHv3] block: introduce BDRV_O_SEQUENTIAL |
Date: |
Wed, 14 May 2014 11:31:11 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 |
On 05/13/2014 06:36 AM, Peter Lieven wrote:
> this patch introduces a new flag to indicate that we are going to sequentially
> read from a file and do not plan to reread/reuse the data after it has been
> read.
>
> The current use of this flag is to open the source(s) of a qemu-img convert
> process. If a protocol from block/raw-posix.c is used posix_fadvise is
> utilized
> to advise to the kernel that we are going to read sequentially from the
> file and a POSIX_FADV_DONTNEED advise is issued after each write to indicate
> that there is no advantage keeping the blocks in the buffers.
>
> Consider the following test case that was created to confirm the behaviour of
> the new flag:
>
> A 10G logical volume was created and filled with random data.
> Then the logical volume was exported via qemu-img convert to an iscsi target.
> Before the export was started all caches of the linux kernel where dropped.
>
> Old behavior:
> - The convert process took 3m45s and the buffer cache grew up to 9.67 GB
> close
> to the end of the conversion. After qemu-img terminated all the buffers
> were
> freed by the kernel.
>
> New behavior with the -N switch:
> - The convert process took 3m43s and the buffer cache grew up to 15.48 MB
> close
> to the end with some small peaks up to 30 MB during the conversion.
>
> Signed-off-by: Peter Lieven <address@hidden>
> ---
> v2->v3: - rebased
> - fixed typo in commit msg [Fam]
> v1->v2: - added test example to commit msg
> - added -N knob to qemu-img
>
> block/raw-posix.c | 14 ++++++++++++++
> include/block/block.h | 1 +
> qemu-img-cmds.hx | 4 ++--
> qemu-img.c | 15 ++++++++++++---
> qemu-img.texi | 9 ++++++++-
> 5 files changed, 37 insertions(+), 6 deletions(-)
Reviewed-by: Eric Blake <address@hidden>
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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