Re: Recover preallocated space after a crash?

[Dave Chinner <david@fromorbit.com>]

On Sat, Nov 07, 2020 at 08:55:50PM +0100, Gionatan Danti wrote:
> Hi list,
> it is my understanding that XFS can preallocate some "extra" space via
> speculative EOF preallocation and speculative COW preallocation.
> 
> During normal system operation, that extra space is recovered after some
> time. But what if system crashes? Can it be even recovered? If so, it is
> done at mount time or via a (more invasive) fsck?

It will be done silently the next time the inode is cycled through
memory via an open()/close() pair as specualtive prealloc is removed
on the final close() of a file.

Alternatively, you can trigger reclaim on the current set of
in-memory inodes by running:

# xfs_spaceman -c "prealloc -m 64k -s" /mnt

to remove speculative preallocations of more than 64k from all
inodes that are in-memory and wait for the operation to complete.

You still need to bring the inodes into memory, so you can do this
via find command that reads some inode metadata (e.g. find /mnt
-ctime 2>&1 /dev/null). This means you don't need to actually
open/close each inode in userspace - the filesystem will traversal
all the in-memory inodes and clear the prealloc space itself.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com