Re: [btrfs tools] ability to fail a device...

[Hugo Mills <hugo@carfax.org.uk>]

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On Tue, Sep 08, 2015 at 10:33:54PM +0200, Ian Kumlien wrote:
> On 8 September 2015 at 22:28, Hugo Mills <hugo@carfax.org.uk> wrote:
> > On Tue, Sep 08, 2015 at 02:17:55PM -0600, Chris Murphy wrote:
> >> On Tue, Sep 8, 2015 at 2:13 PM, Ian Kumlien <ian.kumlien@gmail.com> wrote:
> >> > On 8 September 2015 at 22:08, Chris Murphy <lists@colorremedies.com> wrote:
> >> >> On Tue, Sep 8, 2015 at 2:00 PM, Ian Kumlien <ian.kumlien@gmail.com> wrote:
> 
> [--8<--]
> 
> >> > -o degraded should allow readwrite *IF* the data can be written to....
> >> > My question is also, would this keep me from "adding devices"?
> >> > I mean, it did seem like a catch 22 earlier, but that would really
> >> > make a mess of things...
> >>
> >> It is not possible to add a device to an ro filesystem, so effectively
> >> the fs read-writeability is broken in this case.
> >
> >    I thought this particular issue had already been dealt with in 4.2?
> > (i.e. you can still mount an FS RW if it's degraded, but there are
> > still some single chunks on it).
> 
> Single chunks are only on sda - not on sdb...
> 
> There should be no problem...

   The check is more primitive than that at the moment, sadly. It just
checks that the number of missing devices is smaller than or equal to
the acceptable device loss for each RAID profile present on the FS.

> >    Ian: If you can still mount the FS read/write with both devices in
> > it, then it might be worth trying to balance away the problematic
> > single chunks with:
> >
> > btrfs bal start -dprofiles=single -mprofiles=single /mountpoint
> >
> >    Then unmount, pull the dead drive, and remount -odegraded.
> 
> It never completes, too many errors and eventually the disk disappears
> until the machine is turned off and on again... (normal disk reset
> doesn't work)

   The profiles= parameters should limit the balance to just the three
single chunks, and will remove them (because they're empty). It
shouldn't hit the metadata too hard, even if it's raising lots of
errors.

   Hugo.

-- 
Hugo Mills             | The early bird gets the worm, but the second mouse
hugo@... carfax.org.uk | gets the cheese.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

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