Re: Recursive directory accounting for size, ctime, etc.

Sage Weil
2008-08-08T23:32:43Z

On Fri, 8 Aug 2008, John Stoffel wrote:
> >>>>> "Pavel" == Pavel Machek <pavel@suse.cz> writes:
>
> Pavel> On Tue 2008-07-15 11:28:22, Sage Weil wrote:
> >> 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),
> Pavel> ...
>
> >> 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.
>
> Pavel> Having instant rctime would be very nice -- for stuff like locate and
> Pavel> speeding up kde startup.
>
> >> I'm extremely interested in what people think of overloading the file
> >> system interface in this way. Handy? Crufty? Dangerous? Does anybody
>
> Pavel> Too ugly to live.

:)

> Pavel> What about new rstat() syscall?
>
> Or how about tying this into the quotactl() syscall and extending it a
> bit? Say quotactl2(cmd,device,id,addr,path) which is probably just as
> ugly, but seems to make better sense.

Introducing or modifying system calls makes for pretty interfaces, but is
a bit impractical (and overkill) to support something present in only one
filesystem.

So far I think Andreas' suggestion of using pseudo-xattrs is the cleanest
and simplest: it's doesn't interfere with any existing interfaces
(provided the virtual xattr name is well chosen), and is usable via
standard command line tools like getfattr.

sage