[libvirt RFC] qemu_saveimage: only use wrapper when using --bypass-cache

[Daniel P. Berrangé]

On Thu, Apr 21, 2022 at 04:55:53PM +0200, Claudio Fontana wrote:
> a ping on this one;
> 
> this change makes sense to me, and makes performance better in specific cases, by avoiding --bypass-cache before reaching the "cache trashing" state.
> 
> However, before virsh save was guarenteed to go through iohelper, now it is not.
> 
> The iohelper was guaranteeing a virFileDataSync before exiting,
> while after the change the data will still be in-flight between
> the kernel and the device as virsh save returns to the prompt.

This is a correctness problem I'm afraid, notably with network
filesystems

commit f32e3a2dd686f3692cd2bd3147c03e90f82df987
Author: Michal Prívozník <mprivozn at redhat.com>
Date:   Tue Oct 30 19:15:48 2012 +0100

    iohelper: fdatasync() at the end
    
    Currently, when we are doing (managed) save, we insert the
    iohelper between the qemu and OS. The pipe is created, the
    writing end is passed to qemu and the reading end to the
    iohelper. It reads data and write them into given file. However,
    with write() being asynchronous data may still be in OS
    caches and hence in some (corner) cases, all migration data
    may have been read and written (not physically though). So
    qemu will report success, as well as iohelper. However, with
    some non local filesystems, where ENOSPACE is polled every X
    time units, we may get into situation where all operations
    succeeded but data hasn't reached the disk. And in fact will
    never do. Therefore we ought sync caches to make sure data
    has reached the block device on remote host.


It is also a crash-safety issue. ie if the host OS crashes
after we've created the save image, the image will still
exist on disk but can have lost arbitrary amounts of data,
in a way they might not be detectable until the restored
VM randomly crashes hours/days later.


With regards,
Daniel
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