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I have a btrfs seed device, created with mixed data+metadata (mkfs.btrfs --mixed), and also --metadata single. This device is read-only. When doing btrfs device add /dev/sdc1 /mnt, is it possible to disable mixed-mode and to change the metadata profile for this second device? Or do creation options apply to all devices?

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  • @mikeserv I've learned a lot from your pointers and am experimenting with btrfs
    – Anul
    Jun 18, 2015 at 6:06
  • Looks like you can w/ btrfs-balance - though i dunno about getting out --mixed mode. For the profiles its talking about, see man mkfs.btrfs.
    – mikeserv
    Jun 18, 2015 at 6:23
  • But but but!!! If the first mounted device is read-only (as is the case for seeds) btrfs-balance immediately exits with an error.
    – Anul
    Jun 18, 2015 at 6:27
  • I don't think so - the rebalance will be necessary. Unless it might be worked into send/receive or something. I dunno. You're going over my head man - it's been awhile since i got into the fs nitty-gritty.
    – mikeserv
    Jun 18, 2015 at 6:29
  • You gotta umount it first. You can't balance a readonly. Remember you asked why umount the seed? It was because i needed the dev to be replaced so i could remount rw. Once the loop was associated with the array it remained attach to the seed - but umounting then mounting the ram disk enabled the dev to come up rw.
    – mikeserv
    Jun 18, 2015 at 6:30

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man btrfs device reports:

Profile names, used in profiles and convert are one of: raid0, raid1, raid10, raid5, raid6, dup, single. The mixed data/metadata profiles can be converted in the same way, but it’s conversion between mixed and non-mixed is not implemented.

So according to this, you would be unable to change out of mixed mode, though you could change from single to dup (anything else would need more than one writable device).

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