Date: Tue, 5 Aug 2008 20:26:11 +0200
From: Pavel Machek <pavel@suse.cz>
To: Sage Weil <sage@newdream.net>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
       ceph-devel@lists.sourceforge.net
Subject: Re: Recursive directory accounting for size, ctime, etc.
Message-ID: <20080805182610.GD8380@ucw.cz>
References: <Pine.LNX.4.64.0806192313530.7379@cobra.newdream.net>
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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),
...

> 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.

Having instant rctime would be very nice -- for stuff like locate and
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 

Too ugly to live.

What about new rstat() syscall?

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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