On 2018-07-02 13:34, Marc MERLIN wrote:
On Mon, Jul 02, 2018 at 12:59:02PM -0400, Austin S. Hemmelgarn wrote:
Am I supposed to put LVM thin volumes underneath so that I can share
the same single 10TB raid5?
Actually, because of the online resize ability in BTRFS, you don't
technically _need_ to use thin provisioning here. It makes the maintenance
a bit easier, but it also adds a much more complicated layer of indirection
than just doing regular volumes.
You're right that I can use btrfs resize, but then I still need an LVM
device underneath, correct?
So, if I have 10 backup targets, I need 10 LVM LVs, I give them 10%
each of the full size available (as a guess), and then I'd have to
- btrfs resize down one that's bigger than I need
- LVM shrink the LV
- LVM grow the other LV
- LVM resize up the other btrfs
and I think LVM resize and btrfs resize are not linked so I have to do
them separately and hope to type the right numbers each time, correct?
(or is that easier now?)
I kind of linked the thin provisioning idea because it's hands off,
which is appealing. Any reason against it?
No, not currently, except that it adds a whole lot more stuff between
BTRFS and whatever layer is below it. That increase in what's being
done adds some overhead (it's noticeable on 7200 RPM consumer SATA
drives, but not on decent consumer SATA SSD's).
There used to be issues running BTRFS on top of LVM thin targets which
had zero mode turned off, but AFAIK, all of those problems were fixed
long ago (before 4.0).
You could (in theory) merge the LVM and software RAID5 layers, though that
may make handling of the RAID5 layer a bit complicated if you choose to use
thin provisioning (for some reason, LVM is unable to do on-line checks and
rebuilds of RAID arrays that are acting as thin pool data or metadata).
Does LVM do built in raid5 now? Is it as good/trustworthy as mdadm
radi5?
Actually, it uses MD's RAID5 implementation as a back-end. Same for
RAID6, and optionally for RAID0, RAID1, and RAID10.
But yeah, if it's incompatible with thin provisioning, it's not that
useful.
It's technically not incompatible, just a bit of a pain. Last time I
tried to use it, you had to jump through hoops to repair a damaged RAID
volume that was serving as an underlying volume in a thin pool, and it
required keeping the thin pool offline for the entire duration of the
rebuild.
Alternatively, you could increase your array size, remove the software RAID
layer, and switch to using BTRFS in raid10 mode so that you could eliminate
one of the layers, though that would probably reduce the effectiveness of
bcache (you might want to get a bigger cache device if you do this).
Sadly that won't work. I have more data than will fit on raid10
Thanks for your suggestions though.
Still need to read up on whether I should do thin provisioning, or not.
If you do go with thin provisioning, I would encourage you to make
certain to call fstrim on the BTRFS volumes on a semi regular basis so
that the thin pool doesn't get filled up with old unused blocks,
preferably when you are 100% certain that there are no ongoing writes on
them (trimming blocks on BTRFS gets rid of old root trees, so it's a bit
dangerous to do it while writes are happening).
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