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

[Sage Weil]

On Fri, 8 Aug 2008, John Stoffel wrote:
> >>>>> "Pavel" == Pavel Machek <pavel@xxxxxxx> 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
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