On 2015-08-18 13:36, Timothy Normand Miller wrote:
In theory, barring any undiscovered corner cases, yes that will work the way you want it to (assuming that I'm interpreting how you want it to work correctly). As of right now, attributes changed via chattr on subvolumes work just like on directories, so the NOCOW attribute will be inherited by the files created in that directory.Maybe this is a dumb question, but there are always corner cases.I have a subvolume where I want to disable CoW for VM disks. Maybe that's a dumb idea, but that's a recommendation I've seen here and there. Now, in the docs I've seen, +C applies to a directory. Does it apply to subvolumes? And do I apply it to the subvolume within the main volume, or do I apply it to the mount point where I've mounted the subvolume separately? Are there any cases where the flag applies or not depending on how you access the files? The same subvolume for me is accessible via /mnt/btrfs/vms (via the /mnt/btrfs mount point) and /mnt/vms (where the subvolume is mounted). I applied +C to /mnt/btrfs/vms. So what I'm trying to find out is if it also applies when files are accessed via /mnt/vms. Thanks.
One more bit of advice that I don't see mentioned very often, if you have the space, you should pre-allocate the VM disk image files before using them (although make sure it's after the NOCOW attribute is set on them), as this will significantly reduce the degree of fragmentation. If you do decide to do so, fallocate is preferable to dd for this, as it works a lot faster.
Depending on your use case though, I'd almost suggest storing the disk images on another FS still (ext4 would be my recommendation), or if possible, directly on block devices (LVM would be my recommendation in this case, as it still provides the same data migration abilities as btrfs (albeit slightly more convoluted in some cases)). Btrfs has very poor overall performance compared to most other Linux filesystems still (except on SSD's when TRIM is working properly, as the COW nature of btrfs actually helps with the wear-leveling), and this can significantly hurt VM performance.
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