[mythtv-users] 2 TB Hard Drive Recommendations

Simon Hobson linux at thehobsons.co.uk
Mon Dec 6 16:36:13 UTC 2010

Jay Ashworth wrote:

>  > >so two drives out of 6 dying in 18 months, so close to each other
>>  >isn't that great.
>>  That's one of the problems where the idea of RAID breaks down. You
>>  buy a set of disks (same make and model, possibly even from the same
>>  manufacturing batch), stuff them in an enclosure, and run them for
>>  the same power on hours, doing the same usage profile - ie they are
>>  run under near identical conditions. That means the failures are
>>  liable to be less distributed temporally than you might be thinking.
>>  So you've massively increased the chance of two (or more)
>>  simultaneous failures.
>>  I saw that in one of our Unix servers at my last job. A drive failed,
>>  and it took our maintainers a while to get a replacement because they
>>  struggled to understand that they can't supply any old "9GB drive"
>>  and it must be at least the same number of blocks as the smallest in
>>  the array. By the time it did get replaced, two more were showing an
>>  increasing error count and I think we replaced 3 in an array of 4 !
>>  For that reason, I've always setup my arrays to use a bit less than
>>  the full disk to allow for a slightly smaller drive as a replacement.
>If you buy *anything*, hard disks for a RAID, redundant power supplies,
>or what have you, where "pull a unit and replace it" is a fix...
>and you do not buy the spare unit at build time, test it, bag it, and
>stick it on a shelf, you're just fooling yourself.

Well that's one view. An alternative is to note that the MTBF of the 
drives is such that we are *supposed* to have an exceedingly low 
likelyhood of two drives failing within even a few weeks of each 
other. Given that we were *supposed* to have had a hardware 
maintenance to cover all this, "new drive fitted next day" should 
have been adequate. And given the price of those three little letters 
(IBM) on a drive, management didn't want to pay for extras (there 
would have been several spares of different capacities and speeds).

>If you're really motivated about this, you can buy 6 drives for your RAID5,
>and rotate one in every 3 months.  That will spread your failures, too.

There is one downside to that though - it means that each time you 
rotate a drive, you are deliberately downgrading your system (this 
was striped and mirrored for max performance with redundancy) AND you 
are exposing yourself for a time window to a drive failure that takes 
out your array.

You pays your money, and takes you choice !

Brian Wood wrote:

>Also remember that no matter how good your RAID array is, it is *not* a
>backup solution.

Yup, been explaining that to people time and time again over the years.

>But since Myth is only television, I wouldn't worry too much.

**Only TV**
It's far more important than that !

Jay Ashworth wrote:

>If it's not on tape, it's not backed up.

I'll respectfully disagree with that. There are many options that 
constitute a backup, and hard disk *IS* one of them. It fulfills the 
criteria of :
You can put data on it
You can transport it offsite for storage
You can read data back off it later

The point I suspect you'd argue with would be the last point - can 
you still read data after <x> years and <y> journeys by carrier ? 
It's a valid point - and one where you need to take into account your 
requirements. Hard disks should be good for a few years, beyond that 
then you aren't talking backup - but archive which is a different 
matter. Even magtape has storage limitations - don't some outfits 
suggest you need to re-reel tapes every so often to avoid them 
becoming unreadable ?

In fact, there's an argument that archive shouldn't be on magnetic 
(or optical) media. Our oldest readable archives are all on more 
solid media like parchment and stone tablet ;-)

>It's not a backup unless you can make 2 of them, and carry one off site.

I have 4 sets, and keep the latest 2 off-site. Easy enough with hard 
disks, and takes a lot less space than the SLR-100 tapes I used to 
use. I got the SLR drive and a stack of tapes free, but it got to the 
point where home backups were taking days and it would have taken 
days to recover anything in the worst case where I'd have to 
recatalogue the media first.

I now use SATA drives.

>This seems like a good time to link to my piece: "What is the bandwidth
>of a fully laden 747F?"

A point I've made to people in the past - the fastest way to transfer 
data is a van load of disks/tapes/punched tape/whatever. But as 
online gamers will tell you, raw bandwidth isn't everything.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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