> I'm trying to find out how quota is implemented.
> I know about kernel options and such, but can find out if the kernel
> maintains some sort of table in mem or if it's a file...
If I remember correctly, the Linux quota system works (in principle
anyway) a lot like the 4.4BSD quota system. See "The Design and
Implementation of the 4.4BSD Operating System" by McKusick et al [1] if
you want a technical discussion.
Short version: there are two files, "quota.user" and "quota.group" that
store per-user and per-group (respectively) allocated block counts along
with the hard and soft limits. Since updating these every time a user
allocates or frees a block would be slow, the kernel maintains a cache in
memory using the "dquot" structure. This works like any other cache -
when someone's actively allocating/freeing blocks, their dquot is in
memory; after some time passes and/or other people busy the quota system,
the record gets flushed out to the disk files. This is why shutdown
scripts have to disable quotas before they unmount filesystems - disabling
the quota system forces the "cache" to be flushed.
It is, of course, also important that the quota.* files be correct before
the quota subsystem is started, so it knows how much each user/group
already has allocated; if your system crashes without a clean quota
shutdown, the on-disk values may be inconsistent and need to be
regenerated (which is time-consuming for a traditional FS, although a JFS
or LFS can handle this much faster).
Hope that helps!
Adam
[1] - if you want to help out a starving graduate student, buy it from
Amazon.com through my "recommended reading" page:
http://www.cs.bu.edu/students/grads/artdodge/books/
-- Your lives aren't small, but \\ Adam Davenport Bradley, Grad Student you're living them in a small \\ Boston University Computer Science way. Live openly and expansively! \\ artdodge@cs.bu.edu 353-8921/MCS211 II Cor 6:12-13 (The Message) <>< \\ http://www.netwinder.org/~artdodge
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