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

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

10.02.2021 20:11, Max Reitz wrote:
> 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.

OK for me

> 
> 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?
> 

to compare performance for example..


-- 
Best regards,
Vladimir