[linux-lvm] Why doesn't the lvmcache support the discard (trim) command?

[Zdenek Kabelac]

Dne 19. 10. 18 v 11:55 Ilia Zykov napsal(a):
> 
> 
> On 19.10.2018 12:12, Zdenek Kabelac wrote:
>> Dne 19. 10. 18 v 0:56 Ilia Zykov napsal(a):
>>> Maybe it will be implemented later? But it seems to me a little
>>> strange when there is no way to clear the cache from a garbage.
>>> Maybe I do not understand? Can you please explain this behavior.
>>> For example:
>>
>> Hi
>>
>> Applying my brain logic here:
>>
>> Cache (by default) operates on 32KB chunks.
>> SSD (usually) have the minimal size of trimable block as 512KB.
>>
>> Conclusion can be there is non-trivial to even implement TRIM support
>> for cache - as something would need to keep a secondary data structure
>> which would keep the information about which all cached blocks are
>> completely 'unused/trimmed' and available from a 'complete block trim'
>> (i.e. something like when ext4  implements 'fstrim' support.)
>>
>> Second thought -  if there is a wish to completely 'erase' cache - there
>> is very simple path by using 'lvconvert --uncache' - and once the cache
>> is needed again, create cache again from scratch.
>>
>> Note - dm-cache is SLOW moving cache - so it doesn't target acceleration
>> one-time usage - i.e. if you read block just once from slow storage - it
>> doesn't mean it will be immediately cached.
>>
>> Dm-cache is about keeping info about used blocks on 'slow' storage (hdd)
>> which typically does not support/implemnent TRIM. There could be
>> possibly a multi-layer cache, where even the cached device can handle
>> TRIM - but this kind on construct is not really support and it's even
>> unclear if it would make any sense to introduce this concept ATM  (since
>> there would need to be some well measurable benefit).
>>
>> And final note - there is upcoming support for accelerating writes with
>> new dm-writecache target.
>>
>> Regards
>>
>>
>> Zdenek
>>
> 
> Thank you, I supposed it is so.
> One more little question about dm-writecache:
> The description says that:
> 
> "It doesn't cache reads because reads are supposed to be cached in page cache
> in normal RAM."
> 
> Is it only mean, missing reads not promoted to the cache?
> 

Hi

Writecache simply doesn't care about caching your reads at all.
Your RAM with it's page caching mechanism keeps read data as long as there is 
free RAM for this - the less RAM goes to page cache - less read operations 
remains cached.

It's probably worth to add comment about older dm-cache - where read access is 
  basically accounted (so the most used blocks cat be promoted to caching 
storage device) - if the reads are served by your page-cache - they can't be 
accounted - that's just to explain why repeated reads of the same block which 
is basically served by your page-cache doesn't lead to quick promotion of 
block to cache like one could expect without thinking about details behind....


Zdenek