Tuning DD for server image

[Jeff Boyce]

Greetings -

I have been testing methods for making a backup image of the server for my 
small company.  I am limited to using the equipment that I have at hand, 
which consists of a single server (Dell PE 2600, RHEL3 setup as Samba file 
server with a raid5), a desktop connected to the network, and a portable USB 
hard drive.  My objective is to have a complete image of the entire server 
onto the portable USB drive (not just Samba shares) that can be restored 
onto the server with minimum additional tweaking; if a restoration were 
required.  I am preparing to complete some firmware upgrades soon, and would 
like a good image of server just in case (I have tape backups of data 
directories).  I have tried G4L and it has not been successful, and I am 
aware of Mondo, but I don't have equipment to test it with.  I have settled 
on using dd and sending it across the network to the portable USB hard drive 
mounted on the desktop.  The server has an old usb connection so it would 
take about 30 plus hours to transfer an image if the portable USB drive is 
directly connected to it.

My basic usage of dd for this is  dd if=/dev/sda of=/dev/sdb bs=xxxx 
conv=noerror,sync

I have tried a bs=8192 and piping it using netcat to the portable USB drive 
without success (could not see all the partitions when viewing the drive in 
GParted, nor mount anything.  Is it an inappropriate block size that messed 
up this transfer?).  I have tried not specifying a bs=xxxx and sent it 
through ssh with limited success (could see all the partitions, however 
GParted identified bad superblock errors).  I tried specifying a bs=512 and 
sending it through ssh, and this eliminated the superblock errors, however 
subsequently booting from the USB would hang with a kernel panic.

All of these block size parameters have been suggested to me by others, and 
I am not sure how I would determine what would be the best for me in this 
situation.  I did a lot of reading on the net this weekend about this and 
could not come to any good understanding (other than it is a speed vs. 
accuracy trade off).  I am not that concerned about minor variations in 
potential speed across the network because I know that sending it across our 
network to the USB is going to be far faster than directly connecting the 
USB to the server (my estimates this weekend show about 6MB/sec vs. direct 
connection of 1MB/sec).  Can someone explain to me what would be a good 
block size parameter for me in this situation; or better yet, explain to me 
how I could understand and determine what is best for my situation?  Thanks 
for any and all suggestions.

Jeff Boyce
www.meridianenv.com