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SubjectRecursive directory accounting for size, ctime, etc.
All-

Ceph is a new distributed file system for Linux designed for scalability
(terabytes to exabytes, tens to thousands of storage nodes), reliability,
and performance. The latest release (v0.3), aside from xattr support and
the usual slew of bugfixes, includes a unique (?) recursive accounting
infrastructure that allows statistics about all metadata nested beneath a
point in the directory hierarchy to be efficiently propagated up the tree.
Currently this includes a file and directory count, total bytes (summation
over file sizes), and most recent inode ctime. For example, for a
directory like /home, Ceph can efficiently report the total number of
files, directories, and bytes contained by that entire subtree of the
directory hierarchy.

The file size summation is the most interesting, as it effectively gives
you directory-based quota space accounting with fine granularity. In many
deployments, the quota _accounting_ is more important than actual
enforcement. Anybody who has had to figure out what has filled/is filling
up a large volume will appreciate how cumbersome and inefficient 'du' can
be for that purpose--especially when you're in a hurry.

There are currently two ways to access the recursive stats via a standard
shell. The first simply sets the directory st_size value to the
_recursive_ bytes ('rbytes') value (when the client is mounted with -o
rbytes). For example (watch the directory sizes),

$ tar jxf linux-2.6.24.3.tar.bz2
$ ls -l
total 8
drwxr-xr-x 1 root root 0 Jul 10 05:30 .
drwxr-xr-x 8 root root 4096 Jul 9 18:21 ..
drwxrwxr-x 1 root root 254025660 Feb 26 00:20 linux-2.6.24.3
$ du -s linux-2.6.24.3/
254237 linux-2.6.24.3/
$ ls -al linux-2.6.24.3/
total 281
drwxrwxr-x 1 root root 254025660 Feb 26 00:20 .
drwxr-xr-x 1 root root 0 Jul 10 05:30 ..
-rw-rw-r-- 1 root root 628 Feb 26 00:20 .gitignore
-rw-rw-r-- 1 root root 3657 Feb 26 00:20 .mailmap
-rw-rw-r-- 1 root root 18693 Feb 26 00:20 COPYING
-rw-rw-r-- 1 root root 92230 Feb 26 00:20 CREDITS
drwxrwxr-x 1 root root 8984828 Feb 26 00:20 Documentation
-rw-rw-r-- 1 root root 1596 Feb 26 00:20 Kbuild
-rw-rw-r-- 1 root root 93957 Feb 26 00:20 MAINTAINERS
-rw-rw-r-- 1 root root 53162 Feb 26 00:20 Makefile
-rw-rw-r-- 1 root root 16930 Feb 26 00:20 README
-rw-rw-r-- 1 root root 3119 Feb 26 00:20 REPORTING-BUGS
drwxrwxr-x 1 root root 44216036 Feb 26 00:20 arch
drwxrwxr-x 1 root root 349137 Feb 26 00:20 block
drwxrwxr-x 1 root root 959654 Feb 26 00:20 crypto
drwxrwxr-x 1 root root 118578205 Feb 26 00:20 drivers
drwxrwxr-x 1 root root 21526882 Feb 26 00:20 fs
drwxrwxr-x 1 root root 27456604 Feb 26 00:20 include
drwxrwxr-x 1 root root 99077 Feb 26 00:20 init
drwxrwxr-x 1 root root 170827 Feb 26 00:20 ipc
drwxrwxr-x 1 root root 2189735 Feb 26 00:20 kernel
drwxrwxr-x 1 root root 679502 Feb 26 00:20 lib
drwxrwxr-x 1 root root 1213804 Feb 26 00:20 mm
drwxrwxr-x 1 root root 12562134 Feb 26 00:20 net
drwxrwxr-x 1 root root 3940 Feb 26 00:20 samples
drwxrwxr-x 1 root root 1105977 Feb 26 00:20 scripts
drwxrwxr-x 1 root root 740395 Feb 26 00:20 security
drwxrwxr-x 1 root root 12888682 Feb 26 00:20 sound
drwxrwxr-x 1 root root 16269 Feb 26 00:20 usr

Note that st_blocks is _not_ recursively defined, so 'du' still behaves as
expected. If mounted with -o norbytes instead, the directory st_size is
the number of entries in the directory.

The second interface takes advantage of the fact (?) that read() on a
directory is more or less undefined. (Okay, that's not really true, but
it used to return encoded dirents or something similar, and more recently
returns -EISDIR. As far as I know, no sane application expects meaningful
data from read() on a directory...) So, assuming Ceph is mounted with -o
dirstat,

$ cat linux-2.6.24.3/
entries: 27
files: 9
subdirs: 18
rentries: 24418
rfiles: 23062
rsubdirs: 1356
rbytes: 254025660
rctime: 1215668428.051898000

Fields prefixed with 'r' are recursively defined, while
entries/files/subdirs is just for the one directory. 'rctime' is the most
recent ctime within the hierarchy, which should be useful for backup
software or anything else scanning the hierarchy for recent changes.

Naturally, there are a few caveats:

- There is some built-in delay before statistics fully propagate up
toward the root of the hierarchy. Changes are propagated
opportunistically when lock/lease state allows, with an upper bound of (by
default) ~30 seconds for each level of directory nesting.

- Ceph internally distinguishes between multiple links to the same file
(there is a single 'primary' link, and then zero or more 'remote' links).
Only the primary link contributes toward the 'rbytes' total.

- The 'rbytes' summation is over i_size, not blocks used. That means
sparse files "appear" larger than the storage space they actually consume.

- Directories don't yet contribute anything to the 'rbytes' total. They
should probably include an estimate of the storage consumed by directory
metadata. For this reason, and because the size isn't rounded up to the
block size, the 'rbytes' total will usually be slightly smaller than what
you get from 'du'.

- Currently no stats for the root directory itself.


I'm extremely interested in what people think of overloading the file
system interface in this way. Handy? Crufty? Dangerous? Does anybody
know of any applications that rely on or expect meaningful values for a
directory's i_size? Or read() a directory?


More information on the recursive accounting at

http://ceph.newdream.net/wiki/Recursive_accounting

and Ceph itself at

http://ceph.newdream.net/

Cheers-
sage


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