Re: [gentoo-user] Re: btrfs fails to balance

[Rich Freeman]

On Mon, Jan 19, 2015 at 11:50 AM, James <wireless@×××××××××××.com> wrote:
> Bill Kenworthy <billk <at> iinet.net.au> writes:
>
> I was wondering what my /etc/fstab should look like using uuids, raid 1 and
> btrfs.

From mine:
/dev/disk/by-uuid/7d9f3772-a39c-408b-9be0-5fa26eec8342          /
         btrfs           noatime,ssd,compress=none
/dev/disk/by-uuid/cd074207-9bc3-402d-bee8-6a8c77d56959          /data
         btrfs           noatime,compress=none

The first is a single disk, the second is 5-drive raid1.

I disabled compression due to some bugs a few kernels ago.  I need to
look into whether those were fixed - normally I'd use lzo.

I use dracut - obviously you need to use some care when running root
on a disk identified by uuid since this isn't a kernel feature.  With
btrfs as long as you identify one device in an array it will find the
rest.  They all have the same UUID though.

Probably also worth nothing that if you try to run btrfs on top of lvm
and then create an lvm snapshot btrfs can cause spectacular breakage
when it sees two devices whose metadata identify them as being the
same - I don't know where it went but there was talk of trying to use
a generation id/etc to keep track of which ones are old vs recent in
this scenario.

>
> Eventually, I want to run CephFS on several of these raid one btrfs
> systems for some clustering code experiments. I'm not sure how that
> will affect, if at all, the raid 1-btrfs-uuid setup.
>

Btrfs would run below CephFS I imagine, so it wouldn't affect it at all.

The main thing keeping me away from CephFS is that it has no mechanism
for resolving silent corruption.  Btrfs underneath it would obviously
help, though not for failure modes that involve CephFS itself.  I'd
feel a lot better if CephFS had some way of determining which copy was
the right one other than "the master server always wins."

-- 
Rich