Re: [Qemu-devel] savevm too slow

[Kevin Wolf <kwolf@redhat.com>]

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

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> 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