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

Alan Brown ajb2 at mssl.ucl.ac.uk
Tue Apr 3 14:55:15 UTC 2012

On 03/04/12 14:28, Steven Whitehouse wrote:

>> Spinning disks are slow to seek, large arrays even more so.
> Large arrays should be much faster, provided the data is in cache.

Or not, when there's a lot of random IO involved and it's not in cache.

I'm talking about arrays such as nexsan atabeasts (a drawer full of sata 

> 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)

It doesn't and there are specific warnings to disable barriers on ext4 
and friends when using it.

Bcache is writethrough by default. Writeback can be enabled but is beta 
quality and I think it would conflict badly with clustered filesystems.

> What do you mean by flashcache? This perhaps:

Facebook's caching implementation which is almost like bcache but much 
simpler in its implementation.

> http://www.netapp.com/uk/products/storage-systems/flash-cache/
> 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,

There are a few SAN-level accelerators but the cost of those things 
starts around $20,000 and climbs from there.

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