Mounting USB media (hint/tip, btrfs)

Some good tips from the btrfs newsgroup for mounting removable media:

> Better yet, if your btrfs is actually on /dev/sdc right now, let’s get that fstab entry mounting it by UUID instead.
> ls -l /dev/disk/by-uuid | grep sdc
> lrwxrwxrwx 1 root root 10 Jan 3 09:40 12345678-9abc0-1234-5678-9a0123456789 -> ../../sdc
> So then:
> # this is not a real UUID, you need to check /dev/disk/by-uuid on your machine for a real UUID
> UUID=12345678-9abc0-1234-5678-9a0123456789 /path/to/mountpoint compress=lzo,noauto,users,user 0 0
> This is EXTRA important with a USB drive, since it’s HIGHLY likely it won’t always be on the same physical devicename.

One other note: in this particular case, you might actually be better served setting compression by mounting the drive normally, then:

cd /path/to/drive
chattr +c . ; chattr +c * ; chattr +c .*

This will set compression on by default for any future files stored on that USB drive, *without* needing any special mount options.

Why might this be a better idea? Well, if it’s a USB drive, presumably you might want to mount it on foreign systems from time to time. This way, even if you mount the drive on a foreign system that doesn’t know anything about your preferences, it will see the +c on the root directory of the drive, and store any new data on the drive compressed.

The only caveat: +c won’t set the compression algorithm to LZO. It’ll be gzip, which is the default algorithm. (And, of course, this won’t compress any EXISTING data already stored there – only NEW data written to it after you set the +c attribute.)

Rather than use a UUID, you can also set a more human readable ‘label’ for the filesystem and then mount using “LABEL=…” rather than the “UUID=…”

eg when creating a btrfs filesystem:

mkfs.btrfs -L label ...

If using multiple removable devices that will all be connected at the same time, then you must use a unique label for each of them and a corresponding line for each in /etc/fstab. If however only ever one device will ever be connected at a time, then you could use the same label for all and just use one mount command or single /etc/fstab entry.

(Or if you are running a desktop, you could simply rely on the various desktop media mount daemons 🙂 )

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