Bill Vermillion wrote:
"Bits dont fail me now!" was what
[EMAIL PROTECTED] muttered as he hastily typed
this on Fri, Aug 19, 2005 at 12:00 :
1. Re: Parking disk drive heads (Glenn Dawson)
The setups for parking heads pretty much went away when the move
from MFM drive to ATA technology.
Well, they are obvioulsy back as the IBM and apple laptops invoke them.
And if your are worried about a shock that could damage the drive,
you will probably lose your computer at the same time.
not evennearly true, but even if it were, I'd rather lase the
laptop and be able to salvage
teh drive than lose it all.
Check the technical specs on current drives. You will see that
most will handled well over 100G shocks when not running, and
usually far over 20G in operational mode. Considering that
20G to the human body usually means death you aren't going to have
to worry about losing drives to operating bumps unless you have a
habit of dropping them in parachutes from airplanes. :-)
this shows a slight misunderstanding of the use of 'G' in describing
such events.
"shock" Gs are not the same as sustained Gs.
If you bang your hand on the table, the cells on the outer side of
your skin go from 10 km/h (~2m/s) to 0km/h in 0.000001m (1/1000th of a mm).
It takes your cells, traveling at 2m/s about 1/500000 the of a second to
cover that distance, so the accelleration is somethign of the order of
2/(1/500000) or, about 1 million Gs. cells a couple of cells in already see
significantly
lower shocks as the external cells distort, absobing energy, and lengthenning
the abount of time that the inner cells have to come to a stop.
it's something like an order of magnitude per cell for the first few cells.
Shock loading for a two hard objects hitting each other,
(e.g. the ground and a laptop) can be in the 100s of thousands of Gs for
parts directly attached, to hundreds of G's on indirectly attached items
to 10s of G's for shock mounted items such as the disk drive.
Moving requirements for survival from 20G to 100G by parking the
heads in flight can greatly increase the chances that the drive survives.
just placing an unmounted drive down on a hard table, even when
not running, can ruin it. We lost hundresds that way at Whistle until
we did a failure analysis. Just placing a rubber mat on the table.
fixed it and instructing the staff to always put the drives on
a soft surface made the problem go right away.
I have ruined drives with a bad bump in the past - but those
were MFM drives - and that happened in the mid-to-late 1980s.
The first HDs I saw could withstand less than 1G in shipping so
the 5.25" Shugart ST-505 drives - $25090 for 5MB [that is MegaByte]
were shipped in foam padded boxes a bit larger than the drive, and
these boxes were suspended by springs from the corners of a much
larger shipping box - in the 18"x18" size category.
IOW - unless you are running some early ATA drives, shocks when
running are something you don't have to really worry about, unless
you plan to shove the entire computer off the desk when it is
running with the power on.
Bill
[The orignal post was in of freebsd-hackers Digest, Vol 126, Issue 5]
_______________________________________________
freebsd-hackers@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
To unsubscribe, send any mail to "[EMAIL PROTECTED]"
