Re: reiser4: FITRIM ioctl -- how to grab the space?

[Edward Shishkin <edward.shishkin@gmail.com>]

On 08/16/2014 01:27 PM, Ivan Shapovalov wrote:
> On Saturday 16 August 2014 at 10:23:08, Edward Shishkin wrote:	
>> [...]
>>>> So I need to find a way to take rwsem in a read-biased mode... Any
>>>> advice is
>>>> accepted, including "give up with adding of yet another lock and go with
>>>> multiple transactions" :)
>>>
>>> IMHO this is too complicated.
>>>
>>> Why don't you want to grab, say, 20M per iteration?
>>> It should work without any problems, just maintain a
>>> counter of blocks allocated in the iteration..
> I see it mostly like a "good to have" thing.


Nop, we discuss the reasonable implementation of the "must have" -
space reservation by the fitrim process..


>   When the system is under load,
> FITRIM implemented in this "simple" way will be ineffective: it will miss
> some blocks that are temporarily allocated (journal, etc.)


It will be perfectly discarded by the realtime discard


>   even though it is
> theoretically possible to stop accepting new transactions, commit all existing
> ones and perform discard while the system is artificially made idle, then
> resume everything.


IMHO complicated and not need.
Anyway, missing 2 blocks per device is not critical.


>
> So I've been trying to check if this is feasible. Looks like it isn't.
>
>> add the counter to the struct reiser4_context and
>> set it to zero at the beginning of every iteration.
>> use get_current_context() to access the counter.
> Is it needed? The simple way is just a loop consisting of
> - grab space
> - allocate blocks forward from start position (initially 0) until end of
>    partition or limit reached
> - update start position to `last allocated block + 1`
> - put allocated space to the delete set
> - mark for force-committing
> - reiser4_txn_restart_current()
> - if we've reached end of partition earlier, break, otherwise loop
>


Ok, it will also work (with more iterations, though),
well, let's do so..

Thanks,
Edward.