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[v14,63/64] ceph: update documentation regarding snapshot naming limitations

Message ID 20220427191314.222867-64-jlayton@kernel.org (mailing list archive)
State New, archived
Headers show
Series ceph+fscrypt: full support | expand

Commit Message

Jeff Layton April 27, 2022, 7:13 p.m. UTC
From: Luís Henriques <lhenriques@suse.de>

Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Luís Henriques <lhenriques@suse.de>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
 Documentation/filesystems/ceph.rst | 10 ++++++++++
 1 file changed, 10 insertions(+)
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diff --git a/Documentation/filesystems/ceph.rst b/Documentation/filesystems/ceph.rst
index 4942e018db85..d487cabe792d 100644
--- a/Documentation/filesystems/ceph.rst
+++ b/Documentation/filesystems/ceph.rst
@@ -57,6 +57,16 @@  a snapshot on any subdirectory (and its nested contents) in the
 system.  Snapshot creation and deletion are as simple as 'mkdir
 .snap/foo' and 'rmdir .snap/foo'.
+Snapshot names have two limitations:
+* They can not start with an underscore ('_'), as these names are reserved
+  for internal usage by the MDS.
+* They can not exceed 240 characters in size.  This is because the MDS makes
+  use of long snapshot names internally, which follow the format:
+  `_<SNAPSHOT-NAME>_<INODE-NUMBER>`.  Since filenames in general can't have
+  more than 255 characters, and `<node-id>` takes 13 characters, the long
+  snapshot names can take as much as 255 - 1 - 1 - 13 = 240.
 Ceph also provides some recursive accounting on directories for nested
 files and bytes.  That is, a 'getfattr -d foo' on any directory in the
 system will reveal the total number of nested regular files and