Backup devices - idea

[Timothy A. Seufert]

At 12:13 PM -0700 6/1/02, bronto wrote:
>Here's an idea - maybe someone with experience can comment on it.
>
>I'm thinking I may be able to get the best of all worlds by using
>removable/swapable hard drives as my media.  They're fast, reusable,
>large capacity, and relatively cheap.  According to my inquiries, about
>as cheap per gigabyte as tapes, so I could even archive them and not
>re-use them if I were so inclined.  My questions are: 1) Reliability -
>in theory if treated with respect they should be as reliable as
>permanently installed HD's, but is that realistic,

You will need to treat them with a lot of respect.  One of the major 
reliability problems for hard drives is improper handling leading to 
eventual failure.  Definitely a lot more susceptible to physical 
impact than tapes.

>2) will they be
>hot-swappable in linux?  Will I have to manually mount the drive
>everytime they are rotated (daily)?  I know they don't have to be on
>Windows or Mac, but Linux? and

The major problem is hardware support for hotswap, the secondary 
problem software support.  If you plan on using IDE drives and a 
built-in IDE controller, think again.  IDE is not designed to hot 
swap and you can kill drives trying to do so.  Furthermore, last time 
I researched the topic, the Linux kernel has no concept of IDE drives 
going away and coming back.

(Some of the removable IDE drive carrier kits claim to provide safe 
hotswap capability, and they may well do so, but the kernel still 
won't know how to handle it.)

SCSI, on the other hand, has officially defined ways for devices and 
host adapters to support hot swap.  These days, almost any Ultra 160 
SCSI drive and/or HA probably has hotswap support (but read docs to 
be sure; most SCSI HDs have extensive technical manuals that can be 
found on the mfr's web site if you dig enough).  The kernel supports 
rescanning SCSI IDs and there is a script out there which automates 
rescanning all SCSI busses (search for rescan-scsi-bus.sh on Google). 
This will be a lot more expensive than you were anticipating however.

The third way (that I know of) which solves both the hardware problem 
and the software problem is to use FireWire.  FireWire is 
hotswappable by design, of course.  The trick on the software side is 
that FireWire storage devices look like SCSI to Linux, even if 
they're really IDE in the end.  (The FireWire storage device protocol 
is nothing more than a way of using FireWire as a transport for SCSI 
commands, so it really is a form of SCSI.)  The IDE-to-FireWire 
boards used to create FireWire hard drives handle the job of 
translating commands and results between the SCSI domain and the IDE 
hard disk drive.

So, your setup would be a removable hard drive frame in a 5.25" 
external FireWire case.  To swap out a drive, you would:

1. Unmount it
2. Unplug the FireWire cable
3. Power down the external drive case
4. Remove the HD
5. Put in a new one
6. Power up the case
7. Plug the FW cable in
8. Rescan the "SCSI" bus
9. Mount the drive

(optionally, after (1) perform a SCSI bus rescan to let the kernel 
know that the drive was removed).

>3) Compatibility - will using these drives throw a curve ball at
>standard backup utilities?

That would depend on the utility you use, I would guess.
-- 
Tim Seufert