Re: [Qemu-devel] [PATCH 25/27] block/parallels: add prealloc-mode and prealloc-size open paramemets

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Apr 28, 2015 at 10:46:58AM +0300, Denis V. Lunev wrote:
> This is preparational commit for tweaks in Parallels image expansion.
> The idea is that enlarge via truncate by one data block is slow. It
> would be much better to use fallocate via bdrv_write_zeroes and
> expand by some significant amount at once.
> 
> Original idea with sequential file writing to the end of the file without
> fallocate/truncate would be slower than this approach if the image is
> expanded with several operations:
> - each image expanding means file metadata update, i.e. filesystem
>   journal write. Truncate/write to newly truncated space update file
>   metadata twice thus truncate removal helps. With fallocate call
>   inside bdrv_write_zeroes file metadata is updated only once and
>   this should happen infrequently thus this approach is the best one
>   for the image expansion
> - tail writes are ordered, i.e. the guest IO queue could not be sent
>   immediately to the host introducing additional IO delays
> 
> This patch just adds proper parameters into BDRVParallelsState and
> performs options parsing in parallels_open.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Roman Kagan <rkagan@parallels.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/parallels.c | 83 +++++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 77 insertions(+), 6 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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