Why updating atime doesn't update ctime?

password636 · 11-29-2011, 12:45 AM

Hi,

ctime is the inode change time. If reading a file, its atime will be updated, which should cause inode member i_atime changed, which is an inode change. So ctime should also be updated. But if I try to ls a directory on redhat, only the directory atime gets updated, not ctime. Why?

THANKS!

Nominal Animal · 11-29-2011, 02:55 AM

Quote:

Originally Posted by password636

If reading a file, its atime will be updated, which should cause inode member i_atime changed, which is an inode change. So ctime should also be updated. But if I try to ls a directory on redhat, only the directory atime gets updated, not ctime. Why?

Because otherwise ctime would be just a copy of atime in practice.

Only changes made to inodes by userspace processes are reflected in ctime.

password636 · 11-29-2011, 03:22 AM

Quote:

Originally Posted by Nominal Animal

Because otherwise ctime would be just a copy of atime in practice.

Only changes made to inodes by userspace processes are reflected in ctime.

ctime changes in much more situations than atime. Any member change of inode would cause ctime change, such as i_mode (by chmod), i_uid, i_gid(by chown), etc.. But for atime, only file content access. So how can say "otherwise ctime would be just a copy of atime in practice"?

Nominal Animal · 12-03-2011, 05:58 AM

Quote:

Originally Posted by password636

ctime changes in much more situations than atime. Any member change of inode would cause ctime change, such as i_mode (by chmod), i_uid, i_gid(by chown), etc.. But for atime, only file content access. So how can say "otherwise ctime would be just a copy of atime in practice"?

Because in practice, files are read often, while ownership and mode changes extremely rarely (less than once per file lifetime on average).

Basically, mtime tells the last time the file contents have changed; atime tells the last time the content has been accessed (except for indexers and other applications that use the O_NOATIME open(2) flag); and ctime tells the last time the file ownership or access mode has been changed. This means that in practice, ctime is almost always older than atime.

If atime change also triggered an ctime change, then all typical files would have ctime == atime.