Re: Any chance to get snapshot-aware defragmentation?

[Omar Sandoval <osandov@osandov.com>]

On Fri, May 18, 2018 at 04:26:16PM -0600, Chris Murphy wrote:
> On Fri, May 18, 2018 at 12:33 PM, Austin S. Hemmelgarn
> <ahferroin7@gmail.com> wrote:
> > On 2018-05-18 13:18, Niccolò Belli wrote:
> >>
> >> On venerdì 18 maggio 2018 19:10:02 CEST, Austin S. Hemmelgarn wrote:
> >>>
> >>> and also forces the people who have ridiculous numbers of snapshots to
> >>> deal with the memory usage or never defrag
> >>
> >>
> >> Whoever has at least one snapshot is never going to defrag anyway, unless
> >> he is willing to double the used space.
> >>
> > With a bit of work, it's possible to handle things sanely.  You can
> > deduplicate data from snapshots, even if they are read-only (you need to
> > pass the `-A` option to duperemove and run it as root), so it's perfectly
> > reasonable to only defrag the main subvolume, and then deduplicate the
> > snapshots against that (so that they end up all being reflinks to the main
> > subvolume).  Of course, this won't work if you're short on space, but if
> > you're dealing with snapshots, you should have enough space that this will
> > work (because even without defrag, it's fully possible for something to
> > cause the snapshots to suddenly take up a lot more space).
> 
> 
> Curiously, snapshot aware defragmentation is going to increase free
> space fragmentation. For busy in-use systems, it might be necessary to
> use space cache v2 to avoid performance problems.
> 
> I forget the exact reason why the free space tree is not the default,
> I think it has to do with missing repair support?

Yeah, Nikolay is working on that.