Re: [PATCH 3/7] block/qcow2: use compressed write cache

[Max Reitz <mreitz@redhat.com>]

On 29.01.21 17:50, Vladimir Sementsov-Ogievskiy wrote:
> Introduce a new option: compressed-cache-size, with default to 64
> clusters (to be not less than 64 default max-workers for backup job).
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   qapi/block-core.json   |  8 +++-
>   block/qcow2.h          |  4 ++
>   block/qcow2-refcount.c | 13 +++++++
>   block/qcow2.c          | 87 ++++++++++++++++++++++++++++++++++++++++--
>   4 files changed, 108 insertions(+), 4 deletions(-)
> 
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 9f555d5c1d..e0be6657f3 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -3202,6 +3202,11 @@
>   #             an image, the data file name is loaded from the image
>   #             file. (since 4.0)
>   #
> +# @compressed-cache-size: The maximum size of compressed write cache in
> +#                         bytes. If positive must be not less than
> +#                         cluster size. 0 disables the feature. Default
> +#                         is 64 * cluster_size. (since 6.0)

Do we need this, really?  If you don’t use compression, the cache won’t 
use any memory, right?  Do you plan on using this option?

I’d just set it to a sane default.

OTOH, “a sane default” poses two questions, namely whether 64 * 
cluster_size is reasonable – with subclusters, the cluster size may be 
rather high, so 64 * cluster_size may well be like 128 MB.  Are 64 
clusters really necessary for a reasonable performance?

Second, I think I could live with a rather high default if clusters are 
flushed as soon as they are full.  OTOH, as I briefly touched on, in 
practice, I suppose compressed images are just written to constantly, so 
even if clusters are flushed as soon as they are full, the cache will 
still remain full all the time.


Different topic: Why is the cache disableable?  I thought there are no 
downsides?

(Not being able to disable it would make the code simpler, hence me asking.)

Max