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Re: [dm-devel] [PATCH for-3.14] dm cache: add total cache blocks to status output



On Jan 9, 2014, at 4:13 PM, Mike Snitzer wrote:

> On Thu, Jan 09 2014 at  4:28pm -0500,
> Brassow Jonathan <jbrassow redhat com> wrote:
> 
>> Yes, that'd be nice if we could have this.
>> 
>> It would be great if chunk_size (i.e. cache block size) was also
>> included somehow.  If I had that, I could calculate the size of the
>> cache using the status output alone.  I won't complain much if it
>> isn't there though, because I can get that from the mapping table.
>> 
>> The reason I like adding the total number of cache blocks is because I
>> /can't/ get that information from either type of status (_INFO or
>> _TABLE) for the cache target.  Instead, I would have to get the cache
>> device from the mapping table and query that device for its size -
>> possible, but the level of indirection is a pain.  As it sits in the
>> kernel today, it seems strange to provide some information, but not
>> enough to fill in the whole picture.
> 
> So ideally you want both the cache blocksize and the metadata blocksize.
> We can easily add them, wouldn't be the end of the world.  What format
> seems best?
> 
> <#used metadata blocks>/<#total metadata blocks> <metadata block size>
> <#used cache blocks>/<#total cache blocks> <cache block size>
> ...
> 
> or:
> 
> <metadata block size> <#used metadata blocks>/<#total metadata blocks>
> <cache block size> <#used cache blocks>/<#total cache blocks>
> ...

Either is good (you posted the 2nd).

Still, I don't give a crap about how much metadata is used; but if you think it's useful to others, then great.  The check you mentioned to warn/error/prevent the user when providing an insufficiently sized metadata area might be nice.


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