On 2015-08-17 19:06, Duncan wrote:
Yeah, I've been ridiculously luck to have not hit _any_ of the raid56 related bugs. In fact the only issue I've had with it was a result of a btrfs interaction with dm-thinp (if dm-thinp isn't set to zero newly allocated blocks, btrfs sometimes loses it's mind during remount, which in turn reminds me that I meant to check if this was fixed or not).Austin S Hemmelgarn posted on Mon, 17 Aug 2015 07:38:13 -0400 as excerpted:I've also found that BTRFS raid5/6 on top of MD RAID0 mitigates (to a certain extent that is) the performance penalty of doing raid5/6 if you aren't on ridiculously fast storage, probably not something that should be used in production yet, but it's how I've got the near-line backups setup on my home server system.As should be clear from my previous posts on the subject, I'm conservative enough not to be comfortable with the btrfs raid56 implementation yet. My recommendation has been, and remains, unless you're deliberately testing it in ordered to help find/report/workout bugs, give it a year after the nominally full implementation (3.19, so until 4.4), before expecting it to be reasonably as stable as the rest of btrfs (which itself isn't fully stable yet). But the almost-released 4.2 does seem to be past the initial nominally btrfs raid56 full-code bugs, and I'd call an intermediate level backup, with working copies in front and itself backed up in back, a reasonable first working (as opposed to testing) deployment.
And the deployment you suggest is ironically how I use it, I've got my root filesystem on btrfs raid1 across 2 SSD's, with a btrfs raid6 on top of LVM single volumes on a set of 4 1TB HDD's as a target for receive (and configured such that I can directly boot any of the backups there), and then store compressed, encrypted tarballs of the Sunday backups on 3 different cloud storage services and an external 4TB HDD (It's wonderful how Gentoo lends itself so well to custom solutions).
Huh, I didn't know that mdraid allowed that, I know dm-raid through LVM doesn't (which in turn is a large part of what caused me to try btrfs raid56 so soon, I had been going to do btrfs raid1 on top of LVM based raid6).And yes, btrfs raid5/6 over mdraid0 would have the same general complementary nature as btrfs raid1/10 over mdraid0.It may also be worth pointing out that BTRFS raid6 lets you use 4 disks minimum, as opposed to most other raid6 implementations that (unnecessarily, as a 4 disk RAID6 is not a degenerate form) require 5.4-device raid6, btrfs and mdraid both allow that, good point. But of course mdraid6 doesn't have the data integrity, only rebuild-parity.
Attachment:
smime.p7s
Description: S/MIME Cryptographic Signature
