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

Ilia Zykov mail at izyk.ru
Fri Oct 19 09:55:09 UTC 2018

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?

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