On 27.01.2016 19:04, Alex Bligh wrote: > This question concerns the safety of rebases on backing disks whilst a VM is > running from a writing disk. > > Assume that we have the following structure: > > QEMU -------> Writing disk > .... > backing disk N > .... > backing disk 1 > .... > backing disk 0 > > If QEMU is not running, I can happily use 'qemu-img rebase' to merge backing > disks 1 ... N (i.e. so backing disk N is now based on backing disk 0). > > Is it safe to do this whilst the server is running using the writing disk > which is based on backing disk N?
Maybe. You would rebase 1 through N into a new file and change the
backing file of the writing disk to the new file; because qemu is not
writing to the backing disks, that should probably work, but it's not
nice and may break horribly without prior notice.
> It is attractive to think it might be, as even if QEMU has a stale view of
> the contents of backing disk N, backing disks N-1 to 1 (inclusive) will still
> be present, QEMU will still keep an fd open for them, and it will (thus)
> still be able to read the correct sectors from them.
>
> If not, do I need to use QMP magic instead?
This is always the best option, and I would strongly recommend doing so.
block-stream or block-commit should be your friend. block-stream streams
data from backing files into backed files, block-commit pushes data from
backed files into backing files; so block-stream transmits data from
bottom to top, and block-commit transmits it from top to bottom.
With block-commit, you can collapse the backing chain to e.g. (from top
to bottom):
Writing disk -> backing disk 1..N -> backing disk 0
like this (I think):
{ 'execute': 'block-commit',
'arguments': { 'device': '$drive',
'base': 'backing disk 1',
'top': 'backing disk N' } }
The "backing disk 1..N" is the file which contained "backing disk 1" and
now contains all the data from disks 1 through N.
With block-stream, you can collapse it into
Writing disk -> backing disk 0
like this:
{ 'execute': 'block-stream',
'arguments': { 'device': '$drive',
'base': 'backing disk 0' } }
Here, the writing disk would contain all the data from backing disks 1
through N. You could omit the "base" option, in which case the whole
backing chain would be collapsed into the writing disk.
Max
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