Re: Guidance on increasing cache_pool_max_chunks for lvmcache

[Zdenek Kabelac <zdenek.kabelac@gmail.com>]

Dne 17. 07. 24 v 13:42 Pickles Creator napsal(a):
> Hi,
> 
> I'm currently evaluating using lvmcache with a local disk as a cache
> for a slower network attached disk, both of these are SSDs. The
> default value for `cache_pool_max_chunks` is 1,000,000 and I'm
> wondering the about the reasoning behind this. Is there degradation if
> the value is increased or has it not been tested?
> 
> With the current limit, a larger cache will be fairly inefficient with
> smaller random reads due to large chunk size being allocated, and the
> warmup of such a cache would take some time.
> 

Hi

The basic idea is that users should not really be creating that large caches 
as it may turn out less efficient to manipulate metadata of the cached volume.
Although if you have larger machine with a lot of RAM you can increase this 
max value to be i.e. 8 times more.

Other way is to cache bigger chunks  (i.e. 256K) - that should also reduce 
burden on the metadata manipulation.

Another thing to note is -  dm-cache is Hot Spot cache - if you would have 
1.000.000 hot spots on your drive - maybe you may consider consider switching 
to NVMe for the whole drive instead of caching it.

With all that said - if you have a lot of RAM - raising the limit of 
cache_pool_max_chunks is the easy way forward - but it's most likely not the 
best utilization of the caching resource   (aka having  'chunks' in cache 
which are there occupying the space for days/months without any use...)

Regards

Zdenek