[linux-lvm] How to calculate the LVM SSD cache size?

[Brassow Jonathan]

On Oct 13, 2014, at 7:20 AM, Kees de Jong <keesdejong at gmail.com> wrote:

> Hi,
> 
> 
> 
> The documentation is not clear about how big the cache size should be
> for optimal performance. It only gives pointers about how big the meta
> data for the cache should be.
> 
> "Create the cache metadata LV. This LV will hold cache pool metadata.
> The size of this LV should be 1000 times smaller than the cache data
> LV, with a minimum size of 8MiB."
> 
> There is however an example in the man page with a 100GB OriginLV
> which apparently needs (at least) 10GB of cache size. But what is the
> math? I for example have 4 LV's I want to cache. 6GB, 60GB, 100GB and
> 2TB and I have a 240GB SSD which will act as the caching disk.
> 
> Could someone please fill in the gaps here?

It really does depend on your workload.  We don’t have any general advice yet that I can share on how large the overall cache should be relative to the LV it is caching.  (We can compute how much space is needed for the metadata area for a given cache, which is why we can give advice on those numbers.)

You could try out various sizes and see what happens.  You could also do it a bit more deterministically by watching the number of used cache blocks vs the total cache blocks for you workload, as well as checking other reported metrics such as read/write hits/misses.  Currently, this information is had from ‘dmsetup status’.

 brassow