Re: [gentoo-user] planned btrfs conversion: questions

[Marc Joliet]

OK, I read several articles (the LWN.net series from December/January [5] and a
January article from ars technica [6]) and quite a lot of comments on btrfs
today and can answer some of my questions myself.

For completions sake, I also read a zdnet article series (starting at [7]), but
it wasn't quite as good as the other two IMHO.

Am Tue, 6 May 2014 12:18:32 +0200
schrieb Marc Joliet <marcec@×××.de>:

[...]
> This migration will occur in conjunction with a migration of / + /usr to a
> cheap SSD that I just bought (Crucial M500 120 GB). The overall plan is thus as
> follows:
>
> Replace
>
> 	/boot on /dev/md1 (EXT3, RAID 1)
> 	/ (with assorted sub-directories, sans /usr) on /dev/md2 (EXT4, RAID 10)
> 	the rest on LVM on /dev/md3 (all LVs EXT4, RAID 10)
>
> with
>
> 	/ + /boot + /usr + swapfile on the SSD (EXT4)
> 	the rest (/home, my media partitions) on a btrfs RAID 10

This part I think I will stick with.  From what I've read so far, I wouldn't
trust my entire system to btrfs.  Since "the rest" consists of stuff I either
automatically backup (using rsnapshot) or have multiple copies of, I should be
able to recover from a broken btrfs file system fairly easily.

While I am unsure of my choice of RAID level (some comments on LWN.net claim
that the MD RAID 10 is more comparable to btrfs' RAID 1, which I will attempt to
verify myself beforehand). However, due to btrfs' live rebalancing feature, I
worry less about this. By the time I really need more space the RAID 5/6 code
(and maybe N-way mirroring) ought to be stable (or at least finished), or I
can switch to RAID 1 if I need the flexibility.

[...]
> The reason why I would choose EXT4 for the SSD is that btrfs still lacks support
> for swap files and I worry about creating a swap partition on the SSD. Is that
> warranted, or will the wear-levelling of the SSD handle that just fine? Do swap
> partitions support SSDs specially? Also, does anyone know whether EXT4 goes
> beyond "merely" supporting TRIM? That is, the btrfs wiki advertises the
> following:
>
>   "SSD (Flash storage) awareness (TRIM/Discard for reporting free blocks for
>   reuse) and optimizations (e.g. avoiding unnecessary seek optimizations,
>   sending writes in clusters, even if they are from unrelated files. This
>   results in larger write operations and faster write throughput)"
>
> Does EXT4 also implement such optimisations for SSDs?

I will also go ahead with this (despite the open questions), although I will
leave swap on the LVM for now. I think tonight (well, today) I "just" want to
get the SSD running. Furthermore, "btrfs convert" should be able to up-convert
it in the future once btrfs is "production ready" (both articles make a
guesstimate of about 1-2 years).

I think I would also prefer running a few days from the SSD before converting
"the rest" to btrfs, which should be fairly simple at that point.

[...]
> Is btrfs a good choice for / after all?

I have decided: not without a full system backup (which I don't really want).

> And should I be using the most recent
> kernel versions? (I would go with no, despite the advice from upstream, because
> the changes in the last two versions don't seem to be particularly user
> visible, at least to me, from reading kernelnewbies.org.)

I changed my mind on this: I checked the change logs from the btrfs wiki and
realised I should really give the notion of having the latest bug fixes more
weight. *Especially* since the focus of the mainline btrfs development is
stability and performance (and finalisation of central features, e.g., RAID5/6
support, but that's less important).

Thus the question arises: are there any show-stopper bugs in gentoo-sources
3.14.x that I should be aware about? They don't have to be directly btrfs
related.

> I also have a more specific question regarding RAID 10: the btrfs wiki says
> that you can add devices with different sizes to a multiple device setup, but I
> don't think it says to which RAID levels this applies and how. From [0] I would
> say it works with RAID 10 (since that's what the example uses), but thought
> maybe somebody here knows more details and/or gotchas. From my understanding,
> this means that I can iteratively upgrade my RAID 10 to larger drives and have
> btrfs use all of the available space (or at least as much as is possible). This
> is important to me because I currently have 4 320 GB HDDs + 1 (possibly broken,
> must check) spare and wish to be able to upgrade without having to buy four
> HDDs at once.

From the [5] I learned that RAID 10 can be extended with *pairs* of drives, so
that answers that question. Since my SATA ports are all occupied, I can't just
hook up two new drives and remove the old ones.

So I have a new question: does "btrfs replace" work if the new drive is larger
than the old one? Again, according to [0] it sounds like it should work, but
it's not clear to me.

As an addendum: btrfs does not support hot spares, but you can easily replace
one drive with another, so I can keep my current setup; I just have to replace
any failed drive manually (for now).

Also (OT): the possibly broken drive (sda) might have simply been a loose SATA
cable. The first time it spontaneously failed (triggering the minor data loss
mentioned above), adjusting the SATA cables got it to work again, albeit
unstably.  Today I adjusted the SATA cables again after a different drive (sdd)
spontaneously gave up yesterday, and sda started passing SMART tests (both short
and long) again (sdd passed, too). Color me confused :-/ . I should see if I
can buy shorter SATA cables so they don't get in each others way so much.

[...]
> [0]
> https://btrfs.wiki.kernel.org/index.php/SysadminGuide#RAID_and_data_replication
> [1] https://wiki.archlinux.org/index.php/Solid_State_Drives
> [2] https://wiki.archlinux.org/index.php/Btrfs
> [3] http://wiki.gentoo.org/wiki/Btrfs
> [4] http://wiki.gentoo.org/wiki/Btrfs_system_root

[5] http://lwn.net/Articles/576276/
[6]
http://arstechnica.com/information-technology/2014/01/bitrot-and-atomic-cows-inside-next-gen-filesystems/
[7]
http://www.zdnet.com/btrfs-hands-on-my-first-experiments-with-a-new-linux-file-system-7000023681/

> Greetings and thanks in advance for any help given
-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup