[dm-devel] dm thin pool discarding

[Zdenek Kabelac]

Dne 10. 01. 19 v 14:41 Martin Wilck napsal(a):
> On Thu, 2019-01-10 at 12:52 +0100, Zdenek Kabelac wrote:
>> Dne 10. 01. 19 v 12:40 Martin Wilck napsal(a):
>>>
>>    > So far my understanding was that even without zeroing, an LVM
>> thin
>>> volume could be considered as a drive with "discard zeroes data"
>>> property. If there's a flaw in the argument below, please point it
>>> out
>>> to me.
>>>
>>
>> As said - if you discard 'less then aligned' chunk - nothing happens,
>> so it cannot be takes as like it would be always zeroing...
> 
> Yuck, that's the Catch-22 then. Sorry for having missed that on the
> first pass. Wouldn't it be wise to fail discard (or only zeroout?)
> requests which aren't chunk-aligned, rather than just doing nothing?
> 
> I for one would find it very attractive if dm-thin had a mode
> supporting fast zeroout.


I believe '/sys/block/*/queue/discard_zeroes_data  is now always returning 
false for any device as it's been considered as unreliable logic.

So using 'discard' for zeroing is not really an option here.

Zdenek