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

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

On 08/10/2014 08:52 PM, Ivan Shapovalov wrote:
> On Friday 01 August 2014 at 01:19:05, Edward Shishkin wrote:	
>> On 08/01/2014 12:16 AM, Ivan Shapovalov wrote:
>>> [...]
>>> OK. But it still needs to allocate space (even if just to deallocate it
>>> shortly), which means that someone could get ENOSPC while the discard is in
>>> progress, and this ENOSPC will come from the allocator itself.
>>
>> Ah, yes of course, we'll illegally occupy someone's reservation with
>> the following oops...
>>
>>
>>> Do you have any ideas on how to do it gracefully?
>>
>> Define maximal number of allocated blocks in one iteration
>> and reserve this amount of blocks at the beginning of each
>> iteration. Once the limit is exhausted, stop the scan and
>> force to commit the atom.
> This sounds pretty hackish... Isn't there a way to grab all possible space
> at the same time?
> By all possible space I mean (sbinfo->block_count - sbinfo->blocks_used),
> so that `fstrim <mountpoint>` will be efficient even if system is under load
> and atoms are being created continuously.


I am afraid that other processes will return ENOSPC, whereas there is
a lot of free disk space.
Assume fstrim grabbed all possible space. A process X , who needs to
reserve space invokes txnmgr_force_commit_all(). Everyone waits for
commits completion. After this fstrim grabs all possible space again
(there is no any queue for free space reclaimers). Process X returns
ENOSPC.

Edward.