On 1/30/19 2:00 AM, Vladimir Sementsov-Ogievskiy wrote: >> So, I'm trying to test this, and I've discovered something rather >> annoying about persistent snapshots: they DON'T get written to disk >> until the qemu process exits. In other words, even after creating a >> persistent bitmap via QMP (I'm trying to debug my libvirt API for >> incremental snapshots, so I did this via 'virsh snapshot-create-as $dom >> name', but it boils down to a QMP transaction with >> 'block-dirty-bitmap-add' as one of the commands), running: >> >> $ qemu-img info -U Active1.qcow2 >> >> shows >> bitmaps: >> refcount bits: 16 >> >> for as long as the qemu process is running.
> > But what is the benefit of it, except qemu-img info with --force-share option, > which of course is not guaranteed to show valid metadata? > > While qemu is running valid way to obtain info is qmp. Do libvirt call > qemu-img > --fore-share? You're right that libvirt will be using QMP and not qemu-img. But that does not prevent other clients from using qemu-img on a file, regardless of whether the file is in use by a guest. There's also the argument that if qemu dies suddenly (most likely due to a bug or to a host being fenced), knowing what bitmaps were present in the image is better than having nothing at all (but conversely, if qemu dies suddenly, you are likely missing the bitmap data for the most recent changes, so even if disabled bitmaps are flushed and no longer marked in-use, they still won't be sufficient to allow an incremental backup, and a full backup will be necessary regardless of how much or little bitmap information got persisted). While you're right that --force-share is not required to show up-to-date metadata, it also is not required to show nothing at all. And at least knowing that a persistent bitmap is associated with a qcow2 file may make other things obvious - such as the fact that the image can't be resized (until we implement the functionality to support resize and bitmaps in the same image). -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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