[Qemu-devel] savevm too slow

[xuanmao_001]

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

 

so, what the difference between them when using different cache.

 

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.

 

 

thanks.

---

xuanmao_001