Dumping live file systems.

[Graeme Harker]

In article <384 at vela.acs.oakland.edu> schemers at vela.acs.oakland.edu (Roland Schemers III) writes:
> Hello! I have a question about using 'dump'. How dangerious is it to
> dump an active file system? That is, dumping a file system that is
> mounted, and while the system is in multi-user mode? I have gotten
> various replies from DEC, ranging from "It might corrupt the whole dump",
> or "Only the active files will be messed up".
 
The "problem" with dump is that for performance reasons it uses the
raw interface the the disk and thereby skips the file system buffer
and inode cache. Anything in the cache will not be saved by dump.
Notice that doesn't just mean that you will dump "old" versions, you
risk dumping inconsistent files that have been updated in the cache
and only partially flushed to disk. You will see inconsistencies at
block boundaries and bad inode information.
 
Consequently dumping a live file system should be avioded wherever
possible.
 
However in either of the following conditions a live file system can
be dumped with 100% safety:
 
a) the partition has been mounted read-only
 
b) the machine is completely quiet and the buffer cache has been sync-ed
 
If your partition is mounted read-write and you can neither go
single-user nor guarantee a quiet machine, I suggest you pick a time
of the day when the machine is at its quietest and sync the disk
before dumping (and take out an insurance policy).
 
Graeme Harker, Digital SpA, Piazza XX Settembre, 21100 Varese, Italia
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