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authorLukas Czerner <lczerner@redhat.com>2013-04-09 22:11:22 -0400
committerTheodore Ts'o <tytso@mit.edu>2013-04-09 22:11:22 -0400
commit27dd43854227bb0e6ab70129bd21b60d396db2e7 (patch)
tree14e490b9d0ac63583849c4cad4b3ad0123902c3a /Documentation/filesystems
parentf45a5ef91bef7e02149a216ed6dc3fcdd8b38268 (diff)
downloadblackbird-op-linux-27dd43854227bb0e6ab70129bd21b60d396db2e7.tar.gz
blackbird-op-linux-27dd43854227bb0e6ab70129bd21b60d396db2e7.zip
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>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r--Documentation/filesystems/ext4.txt11
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.
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