Subject: Re: [CentOS] Btrfs going forward, was: Errors on
an SSD drive



On Aug 11, 2017, at 11:00 AM, Chris Murphy <[email protected]> wrote:
>
> On Fri, Aug 11, 2017 at 5:41 AM, hw <[email protected]> wrote:
>> That´s one thing I´ve been wondering about: When using btrfs RAID, do you
>> need to somehow monitor the disks to see if one has failed?
>
> Yes.
>
> The block layer has no faulty device handling

That is one of the open questions about Stratis: should its stratisd act in the
place of smartd?

Vote and comment on its GitHub issue here:

https://github.com/stratis-storage/stratisd/issues/72

I’m in favor of it. The daemon had to be there anyway, it makes sense to push
SMART failure indicators up through the block layer into the volume manager
layer so it can react intelligently to the failure, and FreeBSD’s ZFS is
getting such a daemon soon so we want one, too:

https://www.phoronix.com/scan.php?page=news_item&px=ZFSD-For-FreeBSD

>>> Also FWIW Red Hat is deprecating Btrfs, in the RHEL 7.4 announcement.
>>> Support will be removed probably in RHEL 8. I have no idea how it'll
>>> affect
>>> CentOS kernels though. It will remain in Fedora kernels.

I rather doubt btrfs will be compiled out of the kernel in EL8, and even if it
is, it’ll probably be in the CentOSPlus kernels.

What you *won’t* get from Red Hat is the ability to install EL8 onto a btrfs
volume from within Anaconda, the btrfs tools won’t be installed by default, and
if you have a Red Hat subscription, they won’t be all that willing to help you
with btrfs-related problems.

But will you be able to install EL8 onto an existing XFS-formatted boot volume
and mount your old btrfs data volume? I guess “yes.”

I suspect you’ll even be able to manually create new btrfs data volumes in EL8.

>> That would suck badly to the point at which I´d have to look for yet another
>> distribution. The only one ramaining is arch.

openSUSE defaults to btrfs on root, though XFS on /home for some reason:

https://goo.gl/Hiuzbu

>> What do they suggest as a replacement?

Stratis: https://stratis-storage.github.io/StratisSoftwareDesign.pdf

The main downside to Stratis I see is that it looks like 1.0 is scheduled to
coincide with RHEL 8, based on the release dates of RHELs past, which means it
won’t have any kind of redundant storage options to begin with, not even
RAID-1, the only meaningful RAID level when it comes to comparing against btrfs.

The claim is that “enterprise” users don’t want software RAID anyway, so they
don’t need to provide it in whatever version of Stratis ships with EL 8. I
think my reply to that holds true for many of us CentOS users:

https://github.com/stratis-storage/stratis-docs/issues/54

Ah well, my company has historically been skipping even-numbered RHEL releases
anyway due to lack of compelling reasons to migrate from the prior odd-numbered
release still being supported. Maybe Stratis will be ready for prime time by
the time EL9 ships.

>> removing btrfs alltogether would be taking living in the past too
>> many steps too far.

The Red Hat/Fedora developers are well aware that they started out ~7 years
behind when they pushed btrfs forward as a “technology preview” with RHEL 6,
and are now more like 12 years behind the ZFS world after waiting in vain for
btrfs to catch up.

Basically, Stratis is their plan to catch up on the cheap, building atop
existing, tested infrastructure already in Linux.

My biggest worry is that because it’s not integrated top-to-bottom like ZFS is,
they’ll miss out on some of the key advantages you have with ZFS.

I’m all for making the current near-manual LVM2 + MD + DM + XFS lash-up more
integrated and automated, even if it’s just a pretty face in front of those
same components. The question is how well that interface mimics the end user
experience of ZFS, which in my mind still provides the best CLI experience,
even if you compare only on features they share in common. btrfs’ tools are
close, but I guess the correct command much more often with ZFS’ tools.

That latter is an explicit goal of the Stratis project. They know that
filesystem maintenance is not a daily task for most of us, so that we tend to
forget commands, since we haven’t used them in months. It is a major feature
of a filesystem to have commands you can guess correctly based on fuzzy
memories of having used them once months ago.

> Canonical appears to be charging ahead with OpenZFS included by
> default out of the box (although not for rootfs yet I guess)

Correct. ZFS-on-root-on-Ubuntu is still an unholy mess:

https://github.com/zfsonlinux/zfs/wiki/Ubuntu

> I can't tell you for sure what ZoL's faulty device behavior is
> either, whether it ejects faulty or flaky devices and when, or if like
> Btrfs is just tolerates it.

Lacking something like zfsd, I’d guess it just tolerates it, and that you need
to pair it with smartd to have notification of failing devices. You could
script that to have automatic spare replacement.

Or, port FreeBSD’s zfsd over.
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