[Gluster-users] Expanding legacy gluster volumes

James purpleidea at gmail.com
Thu Nov 21 07:38:43 UTC 2013

On Wed, 2013-11-20 at 18:30 +0530, Lalatendu Mohanty wrote:
> On 11/12/2013 05:54 AM, James wrote:
> > Hi there,
> >
> > This is a hypothetical problem, not one that describes specific hardware
> > at the moment.
> >
> > As we all know, gluster currently usually works best when each brick is
> > the same size, and each host has the same number of bricks. Let's call
> > this a "homogeneous" configuration.
> >
> > Suppose you buy the hardware to build such a pool. Two years go by, and
> > you want to grow the pool. Changes in drive size, hardware, cpu, etc
> > will be such that it won't be possible (or sensible) to buy the same
> > exact hardware, sized drives, etc... A heterogeneous pool is
> > unavoidable.
> >
> > Is there a general case solution for this problem? Is something planned
> > to deal with this problem? I can only think of a few specific corner
> > case solutions.
> I am not sure about of issues you are expecting when a heterogeneous 
> configuration is used. As gluster is intelligent enough for handling 
> sub-volumes/bricks with different sizes.  So I think heterogeneous 
> configuration should not be a issue for gluster. Let us know what are 
> the corner cases you have in mind (may be this will give me some 
> pointers to think :)).

I am thinking about performance differences, due to an imbalance of data
stored on type A hosts, versus type B hosts. I am also thinking about
performance simply due to older versus newer hardware. Even at the
interconnect level there could be significant differences (eg: Gigabit
vs. 10gE, etc...)

I'm not entirely sure how well Gluster can keep the data proportionally
balanced (eg: each brick has 60% or 70% free space, independent of
actual Gb stored) if there is a significant enough difference in the
size of the bricks. Any idea?

> > Another problem that comes to mind is ensuring that the older slower
> > servers don't act as bottlenecks to the whole pool
> I think this is unavoidable but the time-line for these kind of change 
> will be around 10 to 15 years. However we can replace bricks if the old 
> servers really slows the whole thing down.

Well I think it's particularly elegant that Gluster works on commodity
hardware, but it would be ideal if it worked with heterogeneous hardware
in a much more robust way. The ideas jdarcy had mentioned seem like they
might solve these problems in a nice way, but afaik they're just ideas
and not code yet.

> > . jdarcy had mentioned
> > that gluster might gain some notion of tiering, to support things like
> > ssd's in one part of the volume, and slow drives at the other end. Maybe
> > this sort of architecture can be used to solve the same problems.
> >
> > Thoughts and discussion welcome.
> >
> > Cheers,
> > James
> >
> >
> >
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