[TriLUG] Re: Dieing hard drive?

[Aaron S. Joyner]

Ben Pitzer wrote:

>Internally mounted IDE drives are not hot swappable.
>
First sentence, untrue.  The proper way to state this would be that most 
on-board IDE controllers are not capable of hot swapping drives.  And 
since most boxen these days use the on-board controller, most internally 
mounted IDE drives can not be swapped.  On the other hand, most on-board 
RAID controllers, or almost all PCI RAID controllers, are capable of 
swapping in and out drives.  Most can even disconnect one drive on a 
channel, while leaving the other drive on the channel sufficiently 
intact.  I have done this numerous times in the BSD world, with generic 
PC hardware, so I know the hardware is capable of it.  It's been a while 
since I've done it in the Linux world, so I can't describe the process 
off the top of my head, but I imagine there are folks on the list who 
can.  In the case of internal drives used for backups, you're often 
going to need to add a controller -- make it a $20 IDE RAID controller, 
and you can easily swap in and out the disks.  Even if they're in a 
drive enclosure which is attached to said controller.  :)

>As Mr. Tate mentioned,
>however, external USB drives would be hot swappable.  Disadvantage:  Slower
>transfer rates, I believe, depending on your system.  I could be wrong about
>that.  I don't know what the transfer rates are for IDE drives much anymore.
>  
>
IDE drives, on a good day, from cache, can transfer 133MB/s over the IDE 
bus.  In practice, you can see sustained read/write times in the 
35-40MB/s range with reasonably good hardware.  USB2 operates at 480Mb/s 
(that's b as in bits, not B as in Bytes), which works out to 60MB/s 
theoretical maximum.  In practice, the USB2 bus is not going to be your 
bottleneck for transfers from a single hard drive.  USB 1.0 on the other 
hand, maxes out at 12Mb/s -- certainly unacceptable for hard drive use.  
Be sure you're using at least a modern 2.4.x kernel and USB 2.0 should 
work "out of the box" with no additional modifications.

>That said, I'm not a huge fan of the spare drive backup plan.  Mainly for
>the reasons that Chip mentioned to Jason.  Hard drives fail much more
>easily, and I trust a tape or optical backup much more because I know that
>if I have to restore, the chances of that backup media failing are pretty
>slim.
>  
>
The nice thing about hard drives failing, is that when they fail, it's 
usually a big noisy "loud" affair (in both environmental and log-based 
logical ways).  Plus, in a smaller system you have to accept a lower 
quality of service due to lower costs.  The saving grace here is that 
it's reasonably unlikely that both of your drives will fail with in 24 
hours of each other - and it's relatively likely that when either one 
starts to fail you'll notice.  You are monitoring the daily output of 
your backup jobs for things like disk errors, aren't you?  :)

>Basically, the fact is that tapes and optical media are more reliable.  We
>don't use them for primary storage for pretty obvious (to me at least)
>reasons.  They're ideal for backups, which is why they're used in enterprise
>backup systems.  Obviously, for my home desktop, I don't need an enterprise
>level system, but I do like the reliability more.
>  
>
Actually, the reason tapes or optical media are used at the enterprise 
level is cost.  The same cost problems that prevent a DLT or AIT system 
from being cost effective are directly related to what makes it very 
cost effective for a large organization.  When you consider the 
difference in archiving 800TB of data on disk, vs 800TBs worth of tapes 
and 10 tape drives... the benefits quickly become clear.  For off-site 
backups there is one additional advantage, in that tape survives transit 
better.  Disks can only sustain a certain amount of shock, which means 
they must be well padded and cared for during transit.  It's also much 
more likely that a business is going to do proper off-site backups than 
any home user, so it's less of a concern that the disks don't sustain 
travel as well.

>Tough call, but I'm still not overly fond of the HDD backup media idea.
>  
>
Just don't discount it with out thoroughly considering the ins and outs 
of the solution.  The solution Enron used for their backup may not be 
the best solution for you, even if it is on sale cheap at liquidation.  :)

Aaron S. Joyner