Re: [dm-devel] [PATCHES] convert dm-thin to use dm-bufio

[Mikulas Patocka <mpatocka redhat com>]

> For me the main attraction to switching to bufio is you don't use
> dm-io.c.
> 
> Hopefully Alasdair will allow you to keep the different memory types;
> I was allocating the buffers in bm either via kmalloc, or pages
> depending on size, but he wanted this changing to a kmem_cache in
> order to 'segregate' the memory.
> 
> I'm not clear how you ascertain the correct working set size for your
> cache, or choose when and how many blocks to flush.  Have you got any
> documentation for this?  thinp shouldn't need a big cache and I'm a
> bit worried a 'grab another buffer if there's memory available' policy
> will make it grow too much.

The working set is 2% of memory or 1/4 of vmalloc space (whichever is 
smaller). This can be changed in 
"/sys/module/dm_bufio/parameters/dm_bufio_cache_size".

Buffers are freed when they are unused for 60 seconds (can be changed in 
"/sys/module/dm_bufio/parameters/dm_bufio_max_age").

Total size is divided by the number of active clients using dm-bufio. So 
that each client gets a fair share.

Writeback is done on commit. When 3/4 of the cache size is used, 
background writeback is started.

Mikulas

> - Joe
>