Re: Feature requests: online backup - defrag - change RAID level

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

On Fri, Sep 13, 2019 at 07:04:28AM -0400, Austin S. Hemmelgarn wrote:
> On 2019-09-12 19:54, Zygo Blaxell wrote:
> > On Thu, Sep 12, 2019 at 06:57:26PM -0400, General Zed wrote:
> > > 
> > > Quoting Chris Murphy <lists@colorremedies.com>:
> > > 
> > > > On Thu, Sep 12, 2019 at 3:34 PM General Zed <general-zed@zedlx.com> wrote:
> > > > > 
> > > > > 
> > > > > Quoting Chris Murphy <lists@colorremedies.com>:
> > > > > 
> > > > > > On Thu, Sep 12, 2019 at 1:18 PM <webmaster@zedlx.com> wrote:
> > > > > > > 
> > > > > > > It is normal and common for defrag operation to use some disk space
> > > > > > > while it is running. I estimate that a reasonable limit would be to
> > > > > > > use up to 1% of total partition size. So, if a partition size is 100
> > > > > > > GB, the defrag can use 1 GB. Lets call this "defrag operation space".
> > > > > > 
> > > > > > The simplest case of a file with no shared extents, the minimum free
> > > > > > space should be set to the potential maximum rewrite of the file, i.e.
> > > > > > 100% of the file size. Since Btrfs is COW, the entire operation must
> > > > > > succeed or fail, no possibility of an ambiguous in between state, and
> > > > > > this does apply to defragment.
> > > > > > 
> > > > > > So if you're defragging a 10GiB file, you need 10GiB minimum free
> > > > > > space to COW those extents to a new, mostly contiguous, set of exents,
> > > > > 
> > > > > False.
> > > > > 
> > > > > You can defragment just 1 GB of that file, and then just write out to
> > > > > disk (in new extents) an entire new version of b-trees.
> > > > > Of course, you don't really need to do all that, as usually only a
> > > > > small part of the b-trees need to be updated.
> > > > 
> > > > The `-l` option allows the user to choose a maximum amount to
> > > > defragment. Setting up a default defragment behavior that has a
> > > > variable outcome is not idempotent and probably not a good idea.
> > > 
> > > We are talking about a future, imagined defrag. It has no -l option yet, as
> > > we haven't discussed it yet.
> > > 
> > > > As for kernel behavior, it presumably could defragment in portions,
> > > > but it would have to completely update all affected metadata after
> > > > each e.g. 1GiB section, translating into 10 separate rewrites of file
> > > > metadata, all affected nodes, all the way up the tree to the super.
> > > > There is no such thing as metadata overwrites in Btrfs. You're
> > > > familiar with the wandering trees problem?
> > > 
> > > No, but it doesn't matter.
> > > 
> > > 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.

> Do you have a source for this claim of a 128MB max extent size?  

	~/linux$ git grep BTRFS.*MAX.*EXTENT
	fs/btrfs/ctree.h:#define BTRFS_MAX_EXTENT_SIZE SZ_128M

Plus years of watching bees logs scroll by, which never have an extent
above 128M in size that contains data.

I think there are a couple of exceptions for non-data-block extent items
like holes.  A hole extent item doesn't have any physical location on
disk, so its size field can be any 64-bit integer.  btrfs imposes no
restriction there.

PREALLOC extents are half hole, half nodatacow extent.  They can be
larger than 128M when they are empty, but when data is written to them,
they are replaced only in 128M chunks.

> Because
> everything I've seen indicates the max extent size is a full data chunk (so
> 1GB for the common case, potentially up to about 5GB for really big
> filesystems)

If what you've seen so far is 'filefrag -v' output (or any tool based
on the FIEMAP ioctl), then you are seeing post-processed extent sizes
(where adjacent extents where begin[n+1] == end[n] are coalesced for
human consumption), not true on-disk and in-metadata sizes.  FIEMAP is
slow and full of lies.

> > 128MB is big enough you're going to be seeking in the middle of reading
> > an extent anyway.  Once you have the file arranged in 128MB contiguous
> > fragments (or even a tenth of that on medium-fast spinning drives),
> > the job is done.
> > 
> > > On my comouter the ENTIRE METADATA is 1 GB. That would be very tolerable and
> > > doable.
> > 
> > You must have a small filesystem...mine range from 16 to 156GB, a bit too
> > big to fit in RAM comfortably.
> > 
> > Don't forget you have to write new checksum and free space tree pages.
> > In the worst case, you'll need about 1GB of new metadata pages for each
> > 128MB you defrag (though you get to delete 99.5% of them immediately
> > after).
> > 
> > > But that is a very bad case, because usually not much metadata has to be
> > > updated or written out to disk.
> > > 
> > > So, there is no problem.
> > > 
> > > 
>