Settling the HD debate WAS: Re: [mythtv-users] A warning about Samsung HDDs

Brady liquidgecka at gmail.com
Wed Nov 2 00:02:46 EST 2005


Oh yea! I didn't mean to poo-poo cache here! In our tests cache makes
HUGE differences in the speed of the drive. Usually the biggest
differences where on multi stage systems that may read/write from many
places at once. We had toe fortune to be able to play with solid state
drives that where basically a 100% cache setup. All reads came from
memory (SDRAM) while all writes went to both. These systems could
offer massive speed while only costing you your limbs. Obviously cache
did the trick here. All I was trying to say before is that cache
actually didn't increase the life of a hard drive. It was
insignificant in the total operating life of the drive.


On 11/1/05, Robert Denier <denier at umr.edu> wrote:
> On Tue, 2005-11-01 at 10:57 -0500, Steve Adeff wrote:
> > On Monday 31 October 2005 15:20, Robert Denier wrote:
> > > I didn't see this mentioned so I thought I'd add it.  Hard drives come
> > > with different amounts of cache ram.  I believe 16MB is about the
> > > largest out now.
> > >
> > > The point being that a larger cache may result in a bit less work for
> > > the moving components which in turn may result in longer life.  I'm not
> > > sure this would matter in practice though.  Still, if your getting a new
> > > drive, I'd look for at least an 8MB cache for performance reasons if
> > > nothing else..
> >
> > in fact all the extra cache does is allow the drive to appear to operate
> > faster. hdparm has a -t and -T tests, one for cache speeds one for actual
> > disk speeds. Since the amount of data being written stays the same there is
>
> Cache is potentially useful against wear and tear if it causes the total
> amount of head motion to be less.  Perhaps the program asks for the
> first 50KB right then, but in a subsequent operation 5 seconds later it
> asks for the next 50KB.  Now the head could be anywhere 5 seconds later
> and have to go back to read that next 50kb.  If, on the other hand, it
> read the next 100KB into cache memory during the initial read, it can
> directly refer to that and save on one positioning of the head.
>
> Does it work exactly like this in practice?  I don't know.  I never
> studied caching algorithms to that degree, and from what someone else
> said, the cache size doesn't seem to affect reliability in practice, and
> in the end those are the results that matter.
>
> Still, caching is an important technology.  No, it doesn't always work,
> but you wouldn't see it all over computer architecture if it didn't have
> some significant benefits...
>
> -Robert
>
> > no real difference in component wear. The platters will be spinning no matter
> > what and the head movement is done electromechanically, so theres no real
> > wear and tear to be concerned about.
>
>
>
>
>
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