On Sun, Nov 23, 2014 at 07:52:29AM +0000, Duncan wrote:
> > Right. So, why would you rebalance empty chunks or near empty chunks?
> > Don't you want to rebalance almost full chunks first, and work you way
> > to less and less full as needed?
>
> No, the closer to empty a chunk is, the more effect you can get in
> rebalancing it along with others of the same fullness.
Ok, now I see what I was thinking the wrong way around:
Rebalancing is not rebalancing data within a chunk, optimizing some tree
data structure.
Rebalancing is taking a nearly empty chunk and merging it with other
chunks to free up that chunkspace.
So, -dusage=10 only picks chunks that are 10% used or less, and tries
to free them up by putting their data elsewhere.
Did I get it right this time? :)
> IOW, rewriting 20 95% usage chunks to 19, freeing just one, is going to
> take you nearly 20 times as long as rewriting 20 5% usage chunks, freeing
> 19 of them, since in the latter case you're actually only rewriting one
> full chunk's worth of data or metadata.
Right, that makes sense.
> OK, so what /is/ the effect of a fuller filesystem? Simply this. As the
> filesystem fills up, there's less and less fully free unallocated space
> available even after a full balance, meaning that free space can be used
> up with fewer and fewer chunk allocations, so you have to rebalance more
> and more often to keep what's left from getting out of balance and
> running into ENOSPC conditions.
Yes, been there, done that :)
> But, beware! Just because your filesystem is say 55% full (number from
> your example earlier), does **NOT** mean usage=55 is the best number to
> use. That may well be the case, or it may not. There's simply no
> necessarily direct correlation in that regard, and a recommended N for
> usage=N cannot be determined without a LOT more use-case information than
> simply knowing the filesystem is at 55% capacity.
Yeah, I remember that. I'm ok with using the same number but I
understand it's not a given that it's the perfect number.
> So the 55% filesystem capacity would probably inform my choice of jumps,
> say 20% at a time, but I'd still start much lower and jump at that 20% or
> so at a time.
That makes sense. I'll try to synthetize all this and rewrite my blog
post and the wiki to make this clearer.
Thanks,
Marc
--
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
.... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/ | PGP 1024R/763BE901
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