Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!bbn!oberon!cit-vax!mangler
From: mangler@cit-vax.Caltech.Edu (System Mangler)
Newsgroups: comp.unix.wizards,comp.unix.questions
Subject: Re: UNIX tape backup utilities ( SysAdmin from UniSolutions Assoc.)
Message-ID: <4040@cit-vax.Caltech.Edu>
Date: Mon, 21-Sep-87 05:44:04 EDT
Article-I.D.: cit-vax.4040
Posted: Mon Sep 21 05:44:04 1987
Date-Received: Tue, 22-Sep-87 01:30:37 EDT
References: <153@tijc02.UUCP> <1143@ttidca.TTI.COM> <358@mcdsun.UUCP> <261@dnlunx.UUCP>
Organization: California Institute of Technology
Lines: 37
Summary: different kinds of verify
Xref: mnetor comp.unix.wizards:4375 comp.unix.questions:4189

In article <160@tijc02.UUCP>, seb022@tijc02.UUCP (Scott Bemis	      ) writes:
> Why can I afford to verify backup tapes on a VAX using the VMS operating
> system,  but may not have the time nor utilites to verify backup tapes on a VAX
> using the UNIX operating system?

Verifying backup tapes can be done to varying degrees, depending upon
the resources you're willing to commit:

    running a tape exerciser when the tape is bought, to reject defectives.
    Does not slow down backups at all.

    reading the tape end-to-end looking for I/O errors.  Can run at
    full speed of tape drive.

    read tape, checking checksums.  Double-buffering can make this
    run near full speed.

    read tape, checking that files have the number of blocks claimed.
    Less amenable to double-buffering.	Still does not catch directory
    inconsistencies, nor does it verify contents.

    Verify that every file on the tape has a parent directory.
    Requires building large data structure while reading tape.
    (E.g. "restore t").

    do backup a second time, and compare block-for-block.  Gets out of
    sync at first difference - not useful for active filesystems such
    as the root.  Parallelism possible, so compare can probably be made
    to run only slightly slower than the real backup.

    do backup a second time, and "diff" against the tape, file by
    file.  Usable on active filesystems.  Inherently sequential.

    Restore backup onto spare partition.  Slow... also does not tell you
    whether the result is equivalent to the filesystem you started with.

Which of these are worth the cost?