Re: [nylug-talk] The Small Company's Guide to Hard Drive Failure and Linux

[alex () pilosoft com]

On Thu, 18 Nov 2004, Dave Aitel wrote:

> One might think you could use dd to duplicate your drive. I initially
> tried this, and my results were not good. I did remember to use dd
> if=/dev/hda of=/dev/hdc conv=noerror (the noerror flag is important).  
> However, this takes forever and a day. Basically it'll take all night.  
> So be prepared for that, even on a small drive and a fast system.  They
> sell devices that can do it faster, I think, but you don't have one, do
> you?
You need to do:
dd if=/dev/hda of=/dev/hdc conv=sync,noerror bs=16384

default blocksize of 512 is too small - doing read/write in 512 byte 
increments is very inefficient. increasing blocksize will however increase 
the size of "bad block", if there's a bad sector anywhere in 8192 byte 
block, you'll get zeros on the output. If there are no bad blocks, dd  
will be flyin' - much faster than any other method, assuming your disk is 
mostly full. If disk is <50% full, dd is slower than tar/untar.

> The other issue with dd is that typically replaces every sector on the
> disk. So you'll need a disk EXACTLY the same size as the previous disk.  
> My disk was one meg smaller (40.0 Meg instead of 40.9 Meg). This was an
> annoying problem.
Since you didn't put sync, noerror means "skip bad blocks on input" - that 
means on output, that block will be missing (and thus filesystem all f'd 
up).

> One thing my lilo did that was weird was rewrite the fstab to use
> "LABEL=/" instead of /dev/hda1. If you happen to be hosted at Pilosoft
> (or another co-lo that is run by someone on the local linux users group
> list - very good idea!) they might jump in and save your butt when you
> load it up and it doesn't work and you're too tired to figure out why.
You are welcome :)

> It's not uncommon for a linux machine not to work properly when you
> reboot. This is because it's probably been a few years since you
> rebooted, and you probably redid a lot of libraries in the meantime,
> some of which arn't in the right places. So after you do all this, it's
> good to test out all your services and make sure they are, in fact,
> doing what you think they are. Your co-lo (whom you bought the computer
> from, most likely) often provides a guarantee of hard drives (Pilosoft
> does). Don't save the 50 bucks by taking them up on this, since that bad
> hard drive still has all your corporate data on it. You should be
> getting swamped with email now, since your mail was down and now it's up
> and the interweb is resending things for you.
I have a policy of rebooting boxes every 6 months for this exact reason. 

It is much better to realize that I forgot to put in something into 
startup scripts when I'm ready to troubleshoot the problems, not right 
after an accidental reboot or power loss event.

-alex

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