xfs: add FITRIM support

[Michael Monnerie]

On Mittwoch, 5. Januar 2011 Lukas Czerner wrote:
> If we
> notice that we are running out of space in advance (how much in
> advance?), we can start trimming smaller chunks, until we reach
> reasonable a reasonable pool of reclaimed space, or until we trim
> the whole device.

Would it be possible that all blocks that have been in use since the 
last FITRIM run can be logged? Like this, we would only need to clean 
those. If you have a 2TB volume, probably only 25% of it have been 
rewritten (=500GB) since the last run, and of that maybe 80% are still 
in use at the time we run FITRIM, so only 100GB would need the cleanup.
Maybe each AG could store a bitmap of written blocks, that are reset by 
a FITRIM run. That could be an asynchronous written bitmap and shouldn't 
disturb performance too much. Maybe it's even only needed to store a bit 
per sunit*swidth blocks, to keep that table small. A mount option could 
be used to enable that feature, so only those which use thin 
provisioning or SSDs or similar devices enable it at wish.

Especially for 100TB size devices that seems like something that should 
be thought of, as maybe if you run FITRIM once a week there, only <10TB 
have been rewritten, if at all, and such a table would boost a FITRIM 
run a lot.

But maybe this is just bullshit of my tired brain, and I'm not a dev so 
I have no idea how hard it would be to implement that.

-- 
mit freundlichen Grüssen,
Michael Monnerie, Ing. BSc

it-management Internet Services: Protéger
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