NBD - was Re: [plug] date and time

[Craig Ringer]

  > Sorry Craig -- didn't see your reply earlier.   Bernd's just brought it
> to my attention.   Certainly looks attractive, will give it a go for 
> sure.   So on the "replay" side when you want to rebuild a server (or 
> workstation) partition I presume you boot from a rescue floppy and 
> reverse the process a la
> cp /dev/st0 /device/and/partition/to/restore ?

That's the idea, yeah. There are complications, for example each 
partition on the new disk needs to be >= the size of the origin 
partition. I've only done it with unmounted reiserfs volumes so far but 
in theory it should work fine with any volume that's been properly 
unmounted.

> Would "dd" have advantages or is "cp" safer because it makes no 
> assumptions about good/bad sectors on the disk?

"dd" doesn't like character devices like tapes. If there's a way to 
somehow "link" a block device to a char device like a tape, I'm 
interested - but that's why I used cp.

$ ls -l /dev/scsi/host0/bus0/target0/lun0/mt
crw-rw----    1 root     tape       9,   0 Jan  1  1970 
/dev/scsi/host0/bus0/target0/lun0/mt

If you're facing bad sectors, its probably too late for your data 
anyway. I don't think either dd or cp will skip bad sectors (leaving a 
hole in the image), both fail instead. Hmm... actually you can instruct 
dd to skip bad sectors but I'm not sure if it'd work over NBD.

I've only used this a few times, so I can't yet say "its great, its 
safe, it works perfectly" only that I've tested it successfully.