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[Qemu-discuss] savevm too slow

From: xuanmao_001
Subject: [Qemu-discuss] savevm too slow
Date: Fri, 6 Sep 2013 09:31:20 +0800

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.


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