ext4: introduce reserved space

Currently in ENOSPC condition when writing into unwritten space, or
punching a hole, we might need to split the extent and grow extent tree.
However since we can not allocate any new metadata blocks we'll have to
zero out unwritten part of extent or punched out part of extent, or in
the worst case return ENOSPC even though use actually does not allocate
any space.

Also in delalloc path we do reserve metadata and data blocks for the
time we're going to write out, however metadata block reservation is
very tricky especially since we expect that logical connectivity implies
physical connectivity, however that might not be the case and hence we
might end up allocating more metadata blocks than previously reserved.
So in future, metadata reservation checks should be removed since we can
not assure that we do not under reserve.

And this is where reserved space comes into the picture. When mounting
the file system we slice off a little bit of the file system space (2%
or 4096 clusters, whichever is smaller) which can be then used for the
cases mentioned above to prevent costly zeroout, or unexpected ENOSPC.

The number of reserved clusters can be set via sysfs, however it can
never be bigger than number of free clusters in the file system.

Note that this patch fixes the failure of xfstest 274 as expected.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 5dd957d8b25b..f7cbf574a875 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -494,6 +494,17 @@ Files in /sys/fs/ext4/<devname>
  session_write_kbytes         This file is read-only and shows the number of
                               kilobytes of data that have been written to this
                               filesystem since it was mounted.
+
+ reserved_clusters            This is RW file and contains number of reserved
+                              clusters in the file system which will be used
+                              in the specific situations to avoid costly
+                              zeroout, unexpected ENOSPC, or possible data
+                              loss. The default is 2% or 4096 clusters,
+                              whichever is smaller and this can be changed
+                              however it can never exceed number of clusters
+                              in the file system. If there is not enough space
+                              for the reserved space when mounting the file
+                              mount will _not_ fail.
 ..............................................................................
 
 Ioctls