[rescue] Mounting and Dumping

[Janet L. Campbell]

On Wed, 14 Jan 2004, Sheldon T. Hall wrote:

> Inconsistent, I can stand,  Unreadable, I can't.  Will the fs activity while
> dumping a mounted filesystem make the dump unusable, or just make a couple
> of the files inconsistent?  What about the tape's directory structure?  Any
> chance of something there that will muck things up?

The dump will be readable in any event.

Basically, dump makes a list of files and directories, dumps that
metadata, and then sequentially goes through the list and processes each 
file.  If files are being written to during the dump, then the writes may 
or may not make it into the dump depending on when dump accessed the 
files.

This gives you a consistency problem.  The dump may not represent the 
exact state of the filesystem at any particular instant in time, instead 
it will have information from a time range.  So, if you were dumping a fs 
with files 'A' and 'Z' that were being written, you might get:

dump makes a file list containing 'A' and 'Z'
dump dumps file 'A'
file 'A' gets rewritten
file 'Z' gets rewritten
dump dumps file 'Z'

which would give you a dump where 'Z' had the rewritten content, and 'A' 
had the original content.  This state never actually existed on the disk.  
Sometimes this is an issue, sometimes it is not.

> When I finally upgrade to Solaris 9, I'll use it.  fssnap doesn't seem to be
> available for Solaris 7.

There are a number of other ways to do this, but they probably won't work
for you.  You can sync a mirror to your existing partition, break the
mirror, and then back up the mirror (which requires extra disk space).  
You can use VXfs checkpoints ($$$).  You can use lockfs to lock write
accesses to the file system during backup, which could inconvenience you.
fssnap was a long time in coming.

> Jochen Kunz suggests the same thing: the Classic isn't as fast at the tape.
> He also suggested disableing compression to help it stream, and I'll try
> that.

It's unfortunately a common problem.  Seek time is murder with lots of 
small files.

One interesting solution is to use an archiver such as Joerg's "star", 
which has a configurable FIFO queue for buffering.  If you have extra 
RAM to use for the buffer then this will definitely help you some, and 
may help you a lot:

http://www.fokus.gmd.de/research/cc/glone/employees/joerg.schilling/private/star.html

-Janet