Re: 4 vol raid5 segfault on device delete

[Duncan <1i5t5.duncan@cox.net>]

Craig Johnson posted on Fri, 16 Aug 2013 11:50:59 -0500 as excerpted:

> I have a 4 device volume with raid5 - trying to remove one of the
> devices (plenty of free space) and I get an almost immediate segfault.
>  Scrub shows no errors, repair show space cache invalid but nothing
> else (I remounted with clear cache to be safe).  Lots of corrupt on bdev
> (for 3 out of 4 drives), but I have no file access issues that I know
> of.  Thanks!

Last I knew (kernel 3.10, where it was introduced, but I haven't seen any 
suggestion that 3.11 fixes all the problems yet), btrfs raid5/6 wasn't 
yet ready for anything like real use yet -- the all-OK code was there, 
but it couldn't yet cope with devices disappearing -- recreating the 
missing content from the checksums didn't yet work.

So "an almost immediate segfault" might be expected if you actually 
remove a device from a btrfs raid5/6, because only the all-OK code is 
actually there, it's writing the checksums but it isn't prepared to 
actually use them yet.

Btrfs raid0/1/10 should be usable, and /reasonably/ stable (for a 
filesystem still under development with bugs actively being fixed with 
each kernel release, that is), however (tho raid1 actually means two-way-
mirror, no matter the number of devices).

FWIW, I'm using btrfs raid1 here, but I have backups both to a second 
btrfs raid1 and to reiserfs (my previous filesystem and what I still use 
on "spinning rust, but it's not suitable for ssds, so I use btrfs on 
them), because btrfs IS still experimental.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman