Re: btrfs on top of bcache on top of dmcrypt on top of md raid5

[Marc MERLIN <marc@merlins.org>]

On Sun, Feb 14, 2016 at 01:43:05PM -0700, Chris Murphy wrote:
> Use all defaults for everything. Anything new by show should do the
> right thing including 4096 byte alignment.
> 
> gargamel:~# cryptsetup luksDump /dev/md8
> [snip]
> Payload offset: 3072
> 
> This is a bit weird because the default is 4096. But because the LUKS
> offset (header + payload + extra unused space) is 2MiB, so it doesn't
> affect alignment. There may be unpatched (fixes not backported) in the
> tools of current long term supported distros, that can cause
> misalignment. Probably top concern would be parted/libparted, which
> would start partition 1 at LBA 63, which is not aligned. The upstream
> tools for a long time now have set partition 1 to LBA 2048, but these
> crusty old unpatched versions just seem to persist like a booger you
> can't flick off. It's really annoying - this idea of "stable bugs"
> that go on and on for a decade.

Indeed. Thankfully my partitions now start at 2048 like you say.

The only thing I did wrong last time (when using bcache) is
md5 - dmcrypt - bcache - btrfs
ssd - dmcrypt /

This was stupid, I needed to do this:
md5  - bcache - dmcrypt - btrfs
ssd /

So I think at this point, just to be future proof, I'm going to add bcache 
on top of all block devices I have, before putting dmcrypt on top, even if I
don't have a cache device.
That way I can later add a cache device without problems.

Without doing that, adding bcache later is a full re-install :(

Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/