Re: xfsdump fails to overwrite stream terminator - dumps lost

[Mandy Kirkconnell <alkirkco@xxxxxxx>]

Jeremy Jackson wrote:
> I have done dumps of the filesystems of 3 machines to a common tape drive,
> an Exabyte EXB-8500.  One machine contains the tape drive, the other two use
> rmt.  Five tapes were filled (5GB each), with a sixth only partly used.
>
> Next I wanted to test overwrite handling, as I was having problems with an
> earlier version of xfsdump.  I started a dump of a 75GB filesystem, without
> using the -o flag.  I began with the partly used sixth tape.  The previously
> used dumps had apparently left a stream terminator at media file 12, and
> this is where the dump began.
>
> After the tape became full, a tape change was requested.  I either felt like
> testing xfsdump, or I wasn't thinking clearly, so I accepted the tape change
> request without changing the tape.  As I watched "xfsdump: preparing drive",
> the tape was rewound, and media files were examined.  I was shocked to see
> it continue dumping at media file 12 *again*.  It found a stream terminator,
> and that was all it was waiting for!
>
> I noticed a bug was fixed in 2.2.13 regarding handling multiple backups to a
> single tape, but I'm not sure what the exact problem was that was fixed.
> The earlier dumps on the tapes were with the older version, 2.2.11-1, and
> the *dump in question* was with 2.2.14-1.

As you suspected, this is caused by the problem that got fixed in 
2.2.13.  It's a nasty one!  Any tape backups written with 2.2.12 or 
earlier are not reliable when multiple dump sessions are involved.

The problem was introduced when xfsdump was ported from IRIX to Linux. 
On IRIX, the TS tape driver sets EOF whether it is to the left OR right 
of a tape mark.  On Linux, the ST tape driver ONLY sets EOF when 
positioned to the right of a tape mark.  On IRIX, the MTBSF,1 (backward 
space 1 filemark) ioctl moves backward along the tape until it finds a 
filemark, then positions the tape to the RIGHT of (after) the filemark. 
 On Linux, the MTBSF,1 ioctl moves back one filemark and positions the 
tape to the LEFT of (before) the filemark.  These differences are very 
important!!

These filemark positioning differences were causing the tape to be 
rewound before each dump and the next dump would begin at the exact same 
start position of the last dump.  The result of this was that each new 
dump would always overwrite the previous dump, so a tape would only ever 
 contain one full (most recent) dump session at a time!!

Please use 2.2.13 or later for any new backups.

Thanks,
Mandy

--
Mandy Kirkconnell
SGI, Storage Software Engineer
alkirkco@xxxxxxx