Dump in multi-user mode?

[Eric Black]

In article <780 at sering.cwi.nl> fmr at cwi.nl (Frank Rahmani) writes:
>> 
>> In article <362 at ntvax.UUCP> chris at ntvax.UUCP (Chris Britton) writes:
>>> What are the consequences of running "dump" in multi-user mode? 
>> 
>> I'd say the consequences are very unpredictable unless you unmount the
>> way. Do you want to have to rely on this type of a backup? I can tell you that
>> the tape will most likely NOT verify.
>Sir,
>you don't tell what system you are working on .Under BSD4.[23] and SunOS3.5
>we do dumps for years in multiuser mode with typically up to 50  users on
>a machine without any problem whatsoever.Also everything is restorable
>without one problem ever as long as you keep in mind that only that
>version of a file is saved that was on disk while dump was accessing it.

Whoa!  You're very lucky if you really have never had any problems with
50 active users munging the file system during a dump.  All the user has to
do is write out new contents of the file which is currently being dumped,
and if the flush to the disk comes in the middle of dumping that file, and
the dump breaks up tape blocks such that data blocks for the file in question
are in different physical tape blocks, and you have a trashed file.

Even if you luck out and get a file backed up intact, either old or
new, on the tape (and that is not guaranteed, nope!), you're increasing
your neck-extension if your users are accessing and modifying multiple
files which must be mutually compatible.  If I've got a database system
which uses muliple UNIX files (even DBM, with separate index and data
files), let alone a database big enough to want to have multiple data
files in one database, then a dump made of the active file system very
likely will contain a garbage database.

I wouldn't want to have to depend on such a backup.  I wouldn't, in any case,
trust it; I'd be forced to verify that the restored data is in fact even
internally consistent, I couldn't be sure that it's either an entire new set
or an entire old set.

Still, it might be better than no backup at all.  Might not, though.

-- 
Eric Black	"Garbage in, Gospel out"
Atherton Technology, 1333 Bordeaux Dr., Sunnyvale, CA, 94089
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   Domainist:	ericb at Atherton.COM