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I would like to use BTRFS to host VMs in VirtualBox to be able to use Snapshots without LVM or such. In theory I don't need CoW
behaviour all the time because of performance reasons and using nodatacow
it seems I don't need to. What I would like to have instead is an approach from my understanding Windows and NTFS seems to be using: Data is changed all the time without CoW
, but if one creates a file system snapshot the current data is kept save within that snapshot by copying data away if it is to be changed in any way. The important thing seems to be that this only happens exactly one time per snapshot and really only for the modified blocks. So after a block to be modified has been copied away at first, all subsequent modifications to the same block are just applied, again without CoW
behaviour.
Besides copying the original block of changed data to some safe place, from a performance perspective this makes a lot of sense to me and I would like to have exactly that behaviour to host my VMs. Those write some data all the time and I simply don't see how I need CoW
behaviour for all those changes.
I only want CoW
after I created a file system snapshot by purpose, e.g. for backup purposes. Afterwards I need CoW
of course to be able to keep my snapshots consistent as long as I need it. But again, even after creation of the snapshots I wouldn't need CoW
for all eternity for all data, but only once for the afterwards changed blocks. All changes after the first one could simply be applied as without any CoW
.
From my understanding of the BTRFS docs, if CoW
happens once to some file, it keeps happening forever. But I might be wrong of course...
So, is what I would like to have possible at all with BTRFS?
Did you actually experience performance problems with CoW or are you concerned about theoretical issues? – Daniel B – 2017-03-15T11:32:14.397
@DanielB I'm just concerned and thinking about things, because one can read very different statements regarding CoW and use cases with VMs or databases. – Thorsten Schöning – 2017-03-15T11:34:48.450
Are you talking about FS or VM snapshots? – Seth – 2017-03-15T11:35:03.400
@Seth file system level snapshots. – Thorsten Schöning – 2017-03-15T11:37:43.303
CoW is a real killer for databases, I agree. However, dynamically growing VM images are subject to fragmentation anyway. – Daniel B – 2017-03-15T11:43:52.947
1Just FYI, this isn't really a function of NTFS. Volume shadow copy is layered above the volume driver and below the file system driver. As such it is file system agnostic. It doesn't care about file system metadata, file contents, or any of that. It's all done in terms of logical blocks (blocks within a volume). – Jamie Hanrahan – 2017-03-15T12:04:16.817