Am 22.02.2013 um 13:15 schrieb Paolo Bonzini <pbonz...@redhat.com>: > Il 22/02/2013 12:06, Stefan Hajnoczi ha scritto: >> 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. > > libiscsi has some nice advantages over the Linux iSCSI initiator. One > of them is that you don't need cache=none, but that mustn't count for > Peter. :) Others are that no privileges are needed to configure it, > that it bypasses the "mandatory" partition scanning in the kernel, and > that iscsi URIs are easier to use than udev stable paths.
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) It seems to me that without SG_IO the implementation of a cache in block/iscsi.c shouldn't be that complicated. Peter > > Paolo >
