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

[Stefan Hajnoczi]

On Fri, Feb 22, 2013 at 08:51:48AM +0100, Peter Lieven wrote:
> is there any project or plans to integrate a readcache in qemu either for 
> iscsi or for the whole block layer?
> 
> I was thinking of 2 options:
> a) assign (up to) a fixed amount of ram for readcaching of a VMs disk I/O. 
> Without SG_IO this seems
> to be quite easy as there are only a handful of operations to intercept.
> b) add SSDs to a Node and allow a VM to use up to X MB of that SSD as 
> read-cache. This should still be
> a benefit if the storage for the VMs is on a NAS.
> 
> I would like to do that independent of the OS for 2 reasons. First with iSCSI 
> the OS
> is not involved. And second I would like to be able to add a memory limit for 
> the cache.

I'm not aware of such an effort.  In general QEMU tries to avoid caching
data because the host OS can already do it (if desired).

The Linux cgroups memory controller can limit file cache size for a
specific process or group of processes.

  http://www.kernel.org/doc/Documentation/cgroups/memory.txt

I suggest using the Linux iSCSI initiator together with cgroups to
achieve what you want.  Adding this functionality to QEMU will cost us
in complexity, data integrity worries, and code maintenance.

Stefan