[Linux-cluster] caching of san devices....

Steven Whitehouse swhiteho at redhat.com
Tue Apr 3 13:28:47 UTC 2012


On Tue, 2012-04-03 at 14:14 +0100, Alan Brown wrote:
> Real Dumb Question[tm] time....
> Has anyone tried putting bcache/flashcache in front of shared storage in 
> a GFS2 cluster (on each node, of course)
> Did it work?
> Should it work?
> Is it safe?
> Are there ways of making it safe?
> Am I mad for thinking about it?
> Rationale:
> Spinning disks are slow to seek, large arrays even more so.
Large arrays should be much faster, provided the data is in cache.

> As soon as there's a significant load on our GFS2 cluster the random io 
> limitations of the SAN hardware become the single most important factor 
> limiting performance.
> Only "so much" ram can be installed in any hardware to increase page and 
> dentry caching before physical limits are hit.
> SSD SAN arrays are hideously expensive and can't always be justified to 
> "the powers that be".
> Universities are always tightly funded, but there are many other 
> entities facing similar problems.
I can't see any mention that bcache supports clusters at all. I don't
think that it is likely to work. Certainly the web page I found suggests
that it doesn't support barriers (silently dropped) but I'm not sure
whether that refers to "real" barriers or the flush based system that we
use now. I'd be very surprised if that would work.

What do you mean by flashcache? This perhaps:


It looks like a hardware implementation of the same thing, and I can't
see anything to suggest that it is cluster aware on a first reading of
the docs,


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