Backup verification

[Leslie Mikesell]

In article <741 at visdc.UUCP> jiii at visdc.UUCP (John E Van Deusen III) writes:

>>>	$ find . -depth -print | cpio -o | tee /dev/ct | sum
>>>	$ cat /dev/ct | sum

>> This won't work unless the the filesystem is read-only, since the
>> read of each file will update the file's atime which will then be
>> different in the next cpio stream.

>You wouldn't use this to verify that a file system had been changed;
>there are much better tools for that. 

Like looking at the ctime or mtimes?  If you reinstall the cpio it will
change the ctimes even though the filesystem contents are really the
same.  If you mv a file to a different name it will change the ctime
but not the mtime (of the file).

>I could be admonished for wasting
>the access time information in making the backup, (use cpio -oa), but
>this example was only for illustration.

No, I was out to lunch on two counts there (as pointed out by C. Cahill)
in that the tee example doesn't actually do 2 cpio's and cpio doesn't
maintain the atime info in the archive anyway - it just sets it to match
the stored mtime when you re-install.
Note however, that using cpio -a will modify the ctime of a file as a
side effect of trying to not modify the atime.  This will prevent any
reasonable use of ctime for incremental backups (which is necessary
if you want to catch mv'ed files or files installed with cpio -im or
anything else that knows how to maintain a file's real mtime.)
The only "clean" way to get that effect is to mount the filesystem
read-only which may not always be possible.

>At first I was hard pressed to think of a good purpose for verifying
>that the byte stream from cpio was identical to the byte stream read
>from the tape at a later time.

I've often wanted to know what files have changed since an archive tape
was made (perhaps to update copies at another site).  You can be fairly
certain by comparing the names and inode info, but if you compress and
uncompress some files you haven't really changed them but the ctimes
will be modified.  The GNUtar file-by-file comparison is a nice option
to have available.

Les Mikesell
  les at chinet.chi.il.us