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Re: [Qemu-devel] [PATCH 2/3] qcow2: add option to clean unused cache ent

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 2/3] qcow2: add option to clean unused cache entries after some time
Date: Thu, 28 May 2015 09:13:37 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 05/28/2015 08:56 AM, Max Reitz wrote:
> On 27.05.2015 11:46, Alberto Garcia wrote:
>> This adds a new 'cache-clean-interval' option that cleans all qcow2
>> cache entries that haven't been used in a certain interval, given in
>> seconds.
>> This allows setting a large L2 cache size so it can handle scenarios
>> with lots of I/O and at the same time use little memory during periods
>> of inactivity.
>> This feature currently relies on MADV_DONTNEED to free that memory, so
>> it is not useful in systems that don't follow that behavior.

>> +++ b/qapi/block-core.json
>> @@ -41,6 +41,10 @@
>>   # @corrupt: #optional true if the image has been marked corrupt;
>> only valid for
>>   #           compat >= 1.1 (since 2.2)
>>   #
>> +# @cache-clean-interval: interval in seconds after which unused L2 and
>> +#                        refcount cache entries are removed. If 0 then
>> +#                        this feature is not enabled (since 2.4)
>> +#
>>   # @refcount-bits: width of a refcount entry in bits (since 2.3)
>>   #
>>   # Since: 1.7
>> @@ -50,7 +54,8 @@
>>         'compat': 'str',
>>         '*lazy-refcounts': 'bool',
>>         '*corrupt': 'bool',
>> -      'refcount-bits': 'int'
>> +      'refcount-bits': 'int',
>> +      'cache-clean-interval': 'int'
>>     } }
> I'm not too happy about making this part of ImageInfoSpecificQCow2. Two
> reasons for this: First, it's eventually part of ImageInfo, which is
> defined as "Information about a QEMU image file", but this option cannot
> be set in the image file itself but is only a run-time option.
> Second, my original goal for ImageInfoSpecific was to provide more
> information through qemu-img info, and this value will look pretty
> strange there.
> I don't know how to resolve this quandary, though. Since qemu cannot
> change this interval by itself, I think not providing an interface for
> reading it is fine. On the other hand, if Eric finds such an interface
> absolutely mandatory, I can't think of a better place to return it than
> here.

Can we mark the parameter optional, and only provide it when it is
non-zero?  That way, qemu-img (which cannot set an interval) will not
report it, and the only time it will appear is if it was set as part of
opening the block device under qemu.

> The only solution which would satisfy both requirements would be another
> structure which contains "online" flags, and thus is not evaluated by
> qemu-img info, but only by QMP commands. But that's ugly.

Yeah, I'm not sure such duplication helps.  I'd still like it reported
somewhere, though, as it is nice to query that a requested setting is
actually working.

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

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