Quoting General Zed <general-...@zedlx.com>:
> Quoting Zygo Blaxell <ce3g8...@umail.furryterror.org>:
>
> > On Thu, Sep 12, 2019 at 08:26:04PM -0400, General Zed wrote:
> > >
> > > Quoting Zygo Blaxell <ce3g8...@umail.furryterror.org>:
> > >
> > > > On Thu, Sep 12, 2019 at 06:57:26PM -0400, General Zed wrote:
> > > > >
> > > > > At worst, it just has to completely write-out "all
> > > > metadata", all the way up
> > > > > to the super. It needs to be done just once, because
what's the point of
> > > > > writing it 10 times over? Then, the super is updated as
> > > > the final commit.
> > > >
> > > > This is kind of a silly discussion. The biggest extent possible on
> > > > btrfs is 128MB, and the incremental gains of forcing 128MB
extents to
> > > > be consecutive are negligible. If you're defragging a 10GB
file, you're
> > > > just going to end up doing 80 separate defrag operations.
> > >
> > > Ok, then the max extent is 128 MB, that's fine. Someone here
> > > previously said
> > > that it is 2 GB, so he has disinformed me (in order to
further his false
> > > argument).
> >
> > If the 128MB limit is removed, you then hit the block group size limit,
> > which is some number of GB from 1 to 10 depending on number of disks
> > available and raid profile selection (the striping raid profiles cap
> > block group sizes at 10 disks, and single/raid1 profiles always use 1GB
> > block groups regardless of disk count). So 2GB is _also_ a valid extent
> > size limit, just not the first limit that is relevant for defrag.
> >
> > A lot of people get confused by 'filefrag -v' output, which coalesces
> > physically adjacent but distinct extents. So if you use that tool,
> > it can _seem_ like there is a 2.5GB extent in a file, but it is really
> > 20 distinct 128MB extents that start and end at adjacent addresses.
> > You can see the true structure in 'btrfs ins dump-tree' output.
> >
> > That also brings up another reason why 10GB defrags are absurd on btrfs:
> > extent addresses are virtual. There's no guarantee that a pair
of extents
> > that meet at a block group boundary are physically adjacent, and after
> > operations like RAID array reorganization or free space defragmentation,
> > they are typically quite far apart physically.
> >
> > > I didn't ever said that I would force extents larger than 128 MB.
> > >
> > > If you are defragging a 10 GB file, you'll likely have to do it
> > > in 10 steps,
> > > because the defrag is usually allowed to only use a limited
amount of disk
> > > space while in operation. That has nothing to do with the extent size.
> >
> > Defrag is literally manipulating the extent size. Fragments and extents
> > are the same thing in btrfs.
> >
> > Currently a 10GB defragment will work in 80 steps, but doesn't
necessarily
> > commit metadata updates after each step, so more than 128MB of temporary
> > space may be used (especially if your disks are fast and empty,
> > and you start just after the end of the previous commit interval).
> > There are some opportunities to coalsce metadata updates, occupying up
> > to a (arbitrary) limit of 512MB of RAM (or when memory pressure forces
> > a flush, whichever comes first), but exploiting those opportunities
> > requires more space for uncommitted data.
> >
> > If the filesystem starts to get low on space during a defrag, it can
> > inject commits to force metadata updates to happen more often, which
> > reduces the amount of temporary space needed (we can't delete
the original
> > fragmented extents until their replacement extent is
committed); however,
> > if the filesystem is so low on space that you're worried about running
> > out during a defrag, then you probably don't have big enough contiguous
> > free areas to relocate data into anyway, i.e. the defrag is
just going to
> > push data from one fragmented location to a different
fragmented location,
> > or bail out with "sorry, can't defrag that."
>
> Nope.
>
> Each defrag "cycle" consists of two parts:
> 1) move-out part
> 2) move-in part
>
> The move-out part select one contiguous area of the disk. Almost any
> area will do, but some smart choices are better. It then moves-out all
> data from that contiguous area into whatever holes there are left empty
> on the disk. The biggest problem is actually updating the metadata,
> since the updates are not localized.
> Anyway, this part can even be skipped.
>
> The move-in part now populates the completely free contiguous area with
> defragmented data.
>
> In the case that the move-out part needs to be skipped because the
> defrag estimates that the update to metatada will be too big (like in
> the pathological case of a disk with 156 GB of metadata), it can
> sucessfully defrag by performing only the move-in part. In that case,
> the move-in area is not free of data and "defragmented" data won't be
> fully defragmented. Also, there should be at least 20% free disk space
> in this case in order to avoid defrag turning pathological.
>
> But, these are all some pathological cases. They should be considered in
> some other discussion.
I know how to do this pathological case. Figured it out!
Yeah, always ask General Zed, he knows the best!!!
The move-in phase is not a problem, because this phase generally affects a
low number of files.
So, let's consider the move-out phase. The main concern here is that the
move-out area may contain so many different files and fragments that the
move-out forces a practically undoable metadata update.
So, the way to do it is to select files for move-out, one by one (or even
more granular, by fragments of files), while keeping track of the size of
the necessary metadata update. When the metadata update exceeds a certain
amount (let's say 128 MB, an amount that can easily fit into RAM), the
move-out is performed with only currently selected files (file fragments).
(The move-out often doesn't affect a whole file since only a part of each
file lies within the move-out area).
