External USB2.0 HDD disconnect

[Lennart Sorensen]

On Tue, Jan 20, 2004 at 01:33:51PM -0500, Madison Kelly wrote:
> Hi all,
> 
>   I am looking into alternatives to tape backup after my last little 
> fiasco and I am ready to settle on an external USB2.0 chassis with 
> removeable HDD carrier trays. Now, I know that with other USB devices 
> like digital cameras and keychain memory sticks I can dismount and then 
> physically disconnect the filesystem from the machine while the computer 
> is on. I am *assuming* I can do that with an external HDD chassis as 
> well but I thought I best turn to TLUG for sage advice lest I be greated 
> by the humbling effect ;).

A USB HD chassis should work.  Just remember that you disconnect the USB
after you umount the drive (if mounted), or making sure the disk is
sync'd.  The USB boxes are pretty cheap now as far as I know.  The HDD
removable trays never made any sense to me, and can only be removed
safely while powered off (or disconnected in the case of USB).

>   Any experiences/comments/horror stories?
> 
> Madison
> 
> PS - My plan is to use drives the same size as the server and to have a 
> script mount the drive and then 'dd' the server's data to the backup 
> hard drive thereby hopefully allowing for baremetal recovery if it 
> becomes so needed in the future. Do I need LVM/snapshot to ensure data 
> consistency or is 'dd' sufficient so long as the users are gone for the day?

If after dd reads block 10000 something goes and changes block 5000, it
won't be in the backup.  If something changes a filesize and appends
some data to a file stored at location 20000, you will still get the
data in dd, but the metadata changes will be missed since they are near
the start of the disk.  dd is NOT how to backup a filesystem that is in
use.  LVM snapshots on the other hand can do a atomic snapshot that
should be as valid as the machine is at that point (same as if it had
instantly lost power at that point).  Databases should recover although
perhaps won't be 100% happy about life.

Lennart Sorensen
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