Am Mon, 28 Nov 2016 01:38:29 +0100 schrieb Ulli Horlacher <frams...@rus.uni-stuttgart.de>:
> On Sat 2016-11-26 (11:27), Kai Krakow wrote: > > > > I have vmware and virtualbox VMs on btrfs SSD. > > > As a side note: I don't think you can use "nodatacow" just for one > > subvolume while the other subvolumes of the same btrfs are mounted > > different. The wiki is just wrong here. > > > > The list of possible mount options in the wiki explicitly lists > > "nodatacow" as not working per subvolume - just globally for the > > whole fs. > > Thanks for pointing this out! > I have misunderstood this, first. You can, however, use chattr to make the subvolume root directory (that one where it is mounted) nodatacow (chattr +C) _before_ placing any files or directories in there. That way, newly created files and directories will inherit the flag. Take note that this flag can only applied to directories and empty (zero-sized) files. That way, you get the intended benefit and your next question applies a little less because: > Ok, then next question :-) > > What is better (for a single user workstation): using mount option > "autodefrag" or call "btrfs filesystem defragment -r" (-t ?) via > nightly cronjob? > > So far, I use neither. When using the above method to make your VM images nodatacow, the only fragmentation issue you need to handle is when doing snapshots. Snapshots are subject to copy-on-write. If you do heavy snapshotting, you'll be getting heavy fragmentation based on the write-patterns. I don't know if autodefrag will handle nodatacow files. You may want to use a dedupe utility after defragmentation, like duperemove (running it manually) or bees (a background daemon also trying to keep fragmentation low). If you are doing no or infrequent snapshots, I won't bother with manual defragging at all for your VM images since you're on SSD. If you aren't going to use snapshots at all, you even won't have to think about autodefrag, tho I still recommend to enable it (see post from Duncan). Manual defrag is a highly write-intensive operation, rewriting multiple gigabytes of data. I strongly recommend against using it on a daily basis for SSD. -- Regards, Kai Replies to list-only preferred. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
