Re: [Qemu-devel] (iSCSI-)Readcache in Qemu?

[Paolo Bonzini]

Il 22/02/2013 13:58, Peter Lieven ha scritto:
> Actually this is a great advantage as it always was a lot of hassle to prevent
> udev from scanning iscsi devices that popped up and maintain all that
> scripting around open-iscsi that was necessary. With libiscsi its much 
> smarter.
> 
> As for caching I actually use write back if the flushes are supported by the 
> drivers.
> With libiscsi I know that a write command succeeds only if it has been 
> successfully
> transferred to the storage (which has battery backup).
> 
> My question was mainly arising from brain storming with some colleagues if a 
> read
> cache could help spare some read IOPS.
> 
> I was thinking if there is an already available library that could be used 
> for this purpose (sth like
> libmemcached)

Ah, so you want a write-through cache in QEMU, and a write-back cache in
the storage.  I can see an advantage over the host page cache is in
migration, because it is easy to invalidate the cache.  With qcow2, it
could help if you have small guests and storage with shared backing
files.  But with SAN as the backend, and all provisioning done there,
I'm not sure.

Perhaps switching to something like gluster would have similar benefits.

Paolo