Am 09.09.2013 um 03:57 hat xuanmao_001 geschrieben: > >> the other question: when I change the buffer size #define IO_BUF_SIZE 32768 > >> to #define IO_BUF_SIZE (1 * 1024 * 1024), the savevm is more quickly. > > > Is this for cache=unsafe as well? > > > Juan, any specific reason for using 32k? I think it would be better to > > have a multiple of the qcow2 cluster size, otherwise we get COW for the > > empty part of newly allocated clusters. If we can't make it dynamic, > > using at least fixed 64k to match the qcow2 default would probably > > improve things a bit. > > with cache=writeback. Is there any risk for setting cache=writeback with > IO_BUF_SIZE 1M ?
No. Using a larger buffer size should be safe. Kevin > ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ > xuanmao_001 > > From: Kevin Wolf > Date: 2013-09-06 18:38 > To: xuanmao_001 > CC: qemu-discuss; qemu-devel; quintela; stefanha; mreitz > Subject: Re: savevm too slow > Am 06.09.2013 um 03:31 hat xuanmao_001 geschrieben: > > Hi, qemuers: > > > > > I found that the guest disk file cache mode will affect to the time of > savevm. > > > > the cache 'writeback' too slow. but the cache 'unsafe' is as fast as it can, > > less than 10 seconds. > > > > here is the example I use virsh: > > @cache with writeback: > > #the first snapshot > > real 0m21.904s > > user 0m0.006s > > sys 0m0.008s > > > > #the secondary snapshot > > real 2m11.624s > > user 0m0.013s > > sys 0m0.008s > > > > @cache with unsafe: > > #the first snapshot > > real 0m0.730s > > user 0m0.006s > > sys 0m0.005s > > > > #the secondary snapshot > > real 0m1.296s > > user 0m0.002s > > sys 0m0.008s > > I sent patches that should eliminate the difference between the first > and second snapshot at least. > > > so, what the difference between them when using different cache. > > cache=unsafe ignores any flush requests. It's possible that there is > potential for optimisation with cache=writeback, i.e. it sends flush > requests that aren't necessary in fact. This is something that I haven't > checked yet. > > > the other question: when I change the buffer size #define IO_BUF_SIZE 32768 > > to #define IO_BUF_SIZE (1 * 1024 * 1024), the savevm is more quickly. > > Is this for cache=unsafe as well? > > Juan, any specific reason for using 32k? I think it would be better to > have a multiple of the qcow2 cluster size, otherwise we get COW for the > empty part of newly allocated clusters. If we can't make it dynamic, > using at least fixed 64k to match the qcow2 default would probably > improve things a bit. > > Kevin
