Dne 10. 01. 19 v 16:23 Martin Wilck napsal(a):
On Thu, 2019-01-10 at 16:08 +0100, Zdenek Kabelac wrote:
Dne 10. 01. 19 v 14:41 Martin Wilck napsal(a):
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.
True. But we have "write_zeroes_max_bytes" instead. In device mapper
terms, it's "num_write_zeroes_bios", which isn't set for dm-thin. With
the logic outlined above, it could be set, AFAICT.
I assume this is valid request for enhancement of thin-pool target.
However this 'WRITE_SAME' or whatever scsi low-level command is that is
basically targeted for thinLV user.
So in this case the most effective 'zeroing' of thinLV areas is its discard.
It'd be interface abuse to use it for intentional slow physical 'zeroing' of
allocated chunks before they are 'returned' to the pool.
On the other hand I can imagine some sort of 'new' flag - that would always
zero chunks that are returned to the thin-pool - or it could be a message send
before final release of thinLV.
So in that case - you would send a dm message 'SECURE_ERASE' (or if there is
some common interface for that) before actual lvremove call and thin-pool
would zero out all exclusively provisioned blocks that would be returned back
to pool.
I guess you can formulate something like this as RFE BZ.
Also note - you can do this operation yourself already today by using
thin-tool and using some scripting-fu skills - but it's pretty hacky....
Zdenek
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