Re: [Qemu-devel] [PATCH 10/20] mirror: add buf-size argument to drive-mirror

[Eric Blake <eblake@redhat.com>]

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On 12/12/2012 06:46 AM, Paolo Bonzini wrote:
> This makes sense when the next commit starts using the extra buffer space
> to perform many I/O operations asynchronously.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block/mirror.c         |  6 +++---
>  block_int.h            |  5 +++--
>  blockdev.c             |  9 ++++++++-
>  hmp.c                  |  2 +-
>  qapi-schema.json       |  5 ++++-
>  qmp-commands.hx        |  4 +++-
>  tests/qemu-iotests/041 | 31 +++++++++++++++++++++++++++++++
>  7 files changed, 53 insertions(+), 9 deletions(-)
> 

> @@ -447,7 +447,7 @@ void mirror_start(BlockDriverState *bs, BlockDriverState *target,
>      s->target = target;
>      s->mode = mode;
>      s->granularity = granularity;
> -    s->buf_size = granularity;
> +    s->buf_size = MAX(buf_size, granularity);

So you silently clamp the buffer size if the user gives something larger
than granularity.

> +++ b/qapi-schema.json
> @@ -1641,6 +1641,9 @@
>  #               are smaller than that, else the cluster size.  Must be a
>  #               power of 2 between 512 and 64M.
>  #
> +# @buf-size: #optional maximum amount of data in flight from source to
> +#            target.

Mention that it was added in 1.4.  Also, is it worth mentioning
reasonable bounds (such as granularity), and whether it must be a power
of two?

>  - "granularity": granularity of the dirty bitmap (json-int, optional)
> +- "buf_size": maximum amount of data in flight from source to target
> +  (json-int, default 10M)

Oh, so unlike granularity, this one does NOT have to be a power of 2.
But why is the default 10M if you clamp it to granularity, which
defaults to 64k?  (and again, something else I have to figure out how to
expose from libvirt)

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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