Re: 3.16 Managed to ENOSPC with <80% used

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

Rich Freeman posted on Fri, 26 Sep 2014 10:18:37 -0400 as excerpted:

> On Thu, Sep 25, 2014 at 5:21 PM, Holger Hoffstätte
> <holger.hoffstaette@googlemail.com> wrote:
>> That's why I mentioned adding a second device - that will immediately
>> allow cleanup with headroom. An additional 8GB tmpfs volume can works
>> wonders.
>>
>>
> If you add a single 8GB tmpfs to a RAID1 btrfs array, is it safe to
> assume that you'll still always have a redundant copy of everything on a
> disk somewhere during the recovery?  Would only a single tmpfs volume
> actually help in this case?  I get a bit nervous about doing a cleanup
> that involves moving metadata to tmpfs of all places, since some kind of
> deadlock/etc could result in unrecoverable data loss.
> 
> Doing the same thing with an actual hard drive would concern me less.

That has been my concern too, and why I'd be leery about using a loopback 
on tmpfs, even for the few minutes (more like seconds since I'm on SSD 
and we /are/ talking memory-backed tmpfs) it'd take to free a minimal 
number of chunks (say usage=2% or 5% or whatever, the smallest number 
that actually frees anything).

With SSD and with backups I'd probably do it, but it's not something I 
could recommend, and I'm not sure I'd do it on slower spinning rust, just 
because the time is longer.

I'd probably use a thumb drive or the like, instead, and would certainly 
recommend that to others, altho if they're comfortable with it and want 
to risk it, a loopback file on tmpfs should work fine, provided the power 
doesn't go out in the middle or something.

-- 
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"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman