Messages in this thread | | | Date | Mon, 13 Oct 2003 09:49:20 +0400 | From | Hans Reiser <> | Subject | Re: ReiserFS patch for updating ctimes of renamed files |
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jw schultz wrote:
>On Sun, Oct 12, 2003 at 01:05:19AM -0500, Alex Adriaanse wrote: > > >>Hi, >> >>I ran into some trouble trying to do incremental backups with GNU tar >>(using --listed-incremental) where renaming a file in between backups would >>cause the file to disappear upon restoration. When investigating the issue >>I discovered that this doesn't happen on ext2, ext3, and tmpfs filesystems >>but only on ReiserFS filesystems. I also noticed that for example ext3 >>updates the affected file's ctime upon rename whereas ReiserFS doesn't, so >>I'm thinking this causes tar to believe that the file existed before the >>first backup was taking under the new name, and as a result it doesn't back >>it up during the second backup. So I believe ReiserFS needs to update >>ctimes for renamed files in order for incremental GNU tar backups to work >>reliably. >> >>I made some changes to the reiserfs_rename function that I *think* should >>fix the problem. However, I don't know much about ReiserFS's internals, and >>I haven't been able to test them out to see if things work now since I can't >>afford to deal with potential FS corruption with my current Linux box. >> >>I included a patch below against the 2.4.22 kernel with my changes. Would >>somebody mind taking a look at this to see if I did things right here (and >>perhaps wouldn't mind testing it out either)? If it works then I (and I'm >>sure others who've experienced the same problem) would like to see the >>changes applied to the next 2.4.x (and 2.6.x?) release. >> >> > >Hmm. I'm conflicted. > >rename(2) manpage: > Any other hard links to the file (as created using > link(2)) are unaffected. > >A change to ctime would affect the other links. > >stat(2) manpage: > The field st_ctime is changed by writing or by > setting inode information (i.e., owner, group, link > count, mode, etc.). > >I am not aware of any field in the inode structure that must >be changed by an atomic rename. Per documentation the only >reason rename should update st_ctime is if it does a >link+unlink sequence which would alter st_nlink briefly. > >On the other hand it does seem to me there ought to be some >record that something about the inode changed. st_ctime would >be the only appropriate indicator. > >rename() SUSv3: > Some implementations mark for update the st_ctime > field of renamed files and some do not. Applications > which make use of the st_ctime field may behave > differently with respect to renamed files unless > they are designed to allow for either behavior. > >So reiserfs is on this point definitely standards conformant >already. A change could at best be seen as an enhancement. > > > > > > thanks Mr. Schultz, you saved us a lot of work in reviewing this issue.
In theory it is cleaner and purer to do it the way we did. In practice, Alex's problem seems like a real one, and I don't know how hard it is to change tar to do the right thing. We'll discuss it in a small seminar today.
-- Hans
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