Re: btrfs on software RAID0

[Marc MERLIN <marc@merlins.org>]

On Mon, May 05, 2014 at 10:51:46PM +0200, john terragon wrote:
> Hi.
> I'm about to try btrfs on an RAID0 md device (to be precise there will
> be dm-crypt in between the md device and btrfs). If I used ext4 I
> would set the stride and stripe_width extended options. Is there
> anything similar I should be doing with mkfs.btrfs? Or maybe some
> mount options beneficial to this kind of setting.

This is not directly an answer to your question, so far I haven't used a
special option like this with btrfs on my arrays although my
undertstanding is that it's not as important as with ext4.

That said, please read
http://marc.merlins.org/perso/btrfs/post_2014-04-27_Btrfs-Multi-Device-Dmcrypt.html

1) use align-payload=1024 on cryptsetup instead of something bigger like
8192. This will reduce write amplification (if you're not on an SSD).

2) you don't need md0 in the middle, crypt each device and then use
btrfs built in raid0 which will be faster (and is stable, at least as
far as we know :) ).

Then use /etc/crypttab or a script like this
http://marc.merlins.org/linux/scripts/start-btrfs-dmcrypt
to decrypt all your devices in one swoop and mount btrfs.

Marc
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