Re: Tux3 Report: How fast can we fail?

[Austin S Hemmelgarn <ahferroin7@xxxxxxxxx>]

On 2015-05-27 11:21, Mosis Tembo wrote:
> On 05/27/2015 04:04 PM, Austin S Hemmelgarn wrote:
> > On 2015-05-27 03:37, Mosis Tembo wrote:
> > > On 05/26/2015 12:03 PM, Pavel Machek wrote:
> > > > > We identified the following quality metrics for this algorithm:
> > > > >
> > > > >   1) Never fails to detect out of space in the front end.
> > > > >   2) Always fills a volume to 100% before reporting out of space.
> > > > >   3) Allows rm, rmdir and truncate even when a volume is full.
> > >
> > > This is definitely nonsense. You can not rm, rmdir and truncate
> > > when the volume is full. You will need a free space on disk to perform
> > > such operations. Do you know why?
> > I assume you are referring either to Tux3 specifically or COW
> > filesystems in general,
>
>
> I am referring to modern file systems with transaction models and
> delayed actions.
> Tux3 is not the case?
In a sensibly designed non-COW filesystem, unlink, truncate, and 
FALLOCATE_FL_{PUNCH_HOLE,COLLAPSE_RANGE} should never need to allocate 
anything.  On well designed COW filesystems, you keep a reserve of space 
that is only available for temporary internal use so that these will 
work even when you report the volume as 100% full so you can free space. 
 This is what BTRFS does (although it doesn't always work because of 
segregating data and metadata), and I believe that tux3 does this also, 
although I don't remember for certain.
> > because you very much _can_ do any of those on any of the non-COW
> > filesystems in the Linux kernel
>
>
> It is simply incorrect. ReiserFS is a counterexample.

Apologies, I didn't know about ReiserFS having issues with that (it's 
the only one that I haven't used, and this is yet another reason I 
probably never will), but I know for a fact that it does work on ext*, 
XFS, and JFS (I'm not entirely certain about OCFS2 and GFS2, and NILFS2 
is technically COW because it's log structured).
> > (I know from experience).  Also, IIRC, it was mentioned somewhere that
> > Tux3 keeps a small reserve of space on the volume for internal
> > operations; and, I would assume that if that is the case, it reports
> > the volume full when everything *except* that reserve of space is
> > used, in which case rm, rmdir, and truncate should work fine when the
> > volume is full.
>
>
> Sorry, I prefer to not manipulate with rumors and assumptions when it comes
> to the review for kernel inclusion.
Entirely understandable.

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