Re: *terrible* speed of savevm/loadvm/delvm

[Avi Kivity <avi@redhat.com>]

Michael Tokarev wrote:
> Somewhere between kvm-75 and kvm-78, the
> mentioned commands has been slowed down
> to insane levels.  By "insane" I mean to
> take about 10 minutes(!) to save/load a
> 128MB RAM/1GB HDD VM's state.  It used
> to require several seconds for much larger
> VMs...
>
> Here's a typical sequence of system calls
> during savevm:
[snip strace]

> As you see, it writes 2 bytes, llseeks to THE SAME
> position, writes next 2 bytes and so on.  This takes
> HUGE amount of time, and can be done, in most cases,
> in a single write without any seeks.
>
> Is it just me or are savevm/loadvm/delvm really THAT
> broken?

It's qcow2 that is broken, with the new default cache=writethrough.  
Does cache=writeback speed things up?

This is probably block reference counts being updated.  There were some 
batching patches posted, but they have not been applied yet, and are 
probably insufficient for savevm.

> And since migration to disk has been removed too,
> there's no way currently to save the VM state...

That's being restored.

-- 
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