[Gluster-devel] Awareness of the disk space available on nodes

Nux! nux at li.nux.ro
Wed Oct 1 13:58:09 UTC 2014

Oh :(
I was so looking forward to this feature.
Let's hope 3.7 will have it then, thanks for clarifying.


Sent from the Delta quadrant using Borg technology!


----- Original Message -----
> From: "Jeff Darcy" <jdarcy at redhat.com>
> To: "Nux!" <nux at li.nux.ro>
> Cc: "Vijay Bellur" <vbellur at redhat.com>, gluster-devel at gluster.org
> Sent: Wednesday, 1 October, 2014 14:52:38
> Subject: Re: [Gluster-devel] Awareness of the disk space available on nodes

>> Awesome, so now we can finally have proper "add nodes as you go"
>> setups without having to rebalance etc. Sweet!
> I hate to be Mr. Negativity, but as the author of that patch I think it
> behooves me to point out that things aren't quite that good yet.  The
> patch doesn't remove the need for rebalancing; it just makes rebalancing
> do something more reasonable.  Let's say that we had two bricks A and B,
> with 1TB each.  They would each receive 50% of new files.  Now we add
> brick C, with 2TB.
> * Until we rebalance, A and B each have 50% of the files.
> * If we rebalance *without the patch*, A/B/C will each have 33% of the
> files.  That's too much for A and B, too little for C.
> * If we rebalance *with the patch*, A and B will each have 25% of the
> files, while C has 50%.  In most cases (e.g. except where this might
> cause excessive network/memory load on C) this is a better result.
> The "no need to rebalance" feature, in which existing files are left
> alone but new files are preferentially directed toward new bricks, is
> not yet implemented.  The basic mechanism is to assign new layouts based
> on each brick's *free* space instead of total space.  That's pretty easy
> to do, but risks creating a new problem.  If we continue to allocate
> more files to the newer bricks even after balance has been restored,
> then we'll start to overload the new bricks.  To avoid this, we need to
> do two things.
> (1) Periodically re-evaluate the divergence between our current layout
> and our ideal, setting a flag when the divergence is too great.
> (2) In the create path, if the flag is set, fix the parent directory
> layout before creating the new file.
> (Note for future implementers: instead of a flag, we could use the same
> "commit hash" technique as in http://review.gluster.org/#/c/7702/)
> As should be clear by now, this will not be a trivial addition.  I think
> it still can - and should - be done for 3.7, but it's not there yet.

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