Re: [Qemu-devel] [PATCHv3] block: introduce BDRV_O_SEQUENTIAL

[Eric Blake]

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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