As discussed with ext4 folks, I will try to make ext4 file system support
directory quota (a.k.a., project id in xfs). For keeping consistency
with xfs's implementation, I did some tests on xfs, and I summarized the
result as following. This will help us understand what we can do and
what we can't do. Please correct me if I miss doing some tests or mis-
I just do some tests about rename/hardlink because they are the key
issue from our discussion.
+ unaccounted dir
x accounted dir
+ -> +: ok
+ -> x: ok
I use strace(1) to lookup which syscall is used, and I found that xfs
will return EXDEV when mv(1) tries to use rename(2) to move a dir from
a unaccounted dir to a accounted dir. Then mv uses creat(2)/read(2)/
write(2) syscalls to move this dir.
x -> +: wrong (design by feature?)
If we move a dir from a accounted dir to a unaccounted dir, the quota is
still accounted. It seems that xfs don't remove project id from this
dir and its subdirs/files on this case.
x -> x: ok
Xfs returns EXDEV error when mv(1) uses rename(2) to move a dir from a
accounted dir to another accounted dir (These dirs have different
project ids). Then mv(1) uses creat(1)/read(1)/write(1) syscalls to
move this dir.
rename + x
+ ok ok (EXDEV)
x wrong ok (EXDEV)
+ -> +: ok
+ -> x: error
Xfs also returns EXDEV error to forbid this operation. So that means
that we don't allow users to do a hardlink for a file from a unaccount
dir to a accounted dir.
x -> +: ok
This operation can be executed and no any error is reported. After that
the quota doesn't be changed. When both of two hardlinks are removed,
the quota will be discharged.
x -> x: error
Xfs returns EXDEV error to forbid this operation.
hardlink + x
+ ok error (EXDEV)
x ok error (EXDEV)
As always, any comment or idea are welcome.