What's special about hybrid hard drives?

--On October 16, 2007 3:33:33 AM -0700 "brian.hannon"
<brian.hannonalum.dartmouth.org> wrote:

> What I don't understand is, what's so special about a flash RAM cache
> that's attached to the hard drive? If an OS writer wanted to incorporate
> a small storage area to save frequently-accessed data so that the hard
> drive could spin down to save energy, why wouldn't the OS just create a
> RAM-disk cache in the RAM of the computer?

All modern OSes already do this, including OS X and Windows. There are a
couple of problems with this:

- with the number of apps people run and the size of files these days the
real ram gets eaten up very quickly. And the real ram is more expensive
than the ram used in hybrid drives.

- A ram cache is basically a passive cache, it doesn't cache anything until
loaded from the (slow) HD on request. On reboot or even a log out/log in
cycle, the cache is purged and doesn't cache again until it can be
populated from the slow HD again. A non-volatile cache can remain across a
reboot and even speed boot times (more necessary in the Windows world, IMO)
a passive cache can't speed boot time.

I believe the difference in boot times is far more than a few seconds,
especially in Windows which typically takes longer than a Mac to boot.

On Oct 16, 2007, at 3:33 AM, brian.hannon wrote:

> I read the article about Seagate's hybrid hard drives, and how they
> require explicit OS support to gain any benefit from the flash RAM
> cache.
>
> What I don't understand is, what's so special about a flash RAM
> cache that's attached to the hard drive? If an OS writer wanted to
> incorporate a small storage area to save frequently-accessed data
> so that the hard drive could spin down to save energy, why wouldn't
> the OS just create a RAM-disk cache in the RAM of the computer?

A RAM disk cache has the wee problem that the data goes away upon
power failure if it hasn't made it to the hard disk. Mac OS (thanks
to the freeBSD underpinnings) does indeed use spare RAM to cache the
hard drive. So do Linux and many other systems. That's why you see
little "free" RAM when in fact there may be lots of available RAM,
reached simply by grabbing some of that cache. But you try to write
through to the drive quickly (and the program ought to be able to
tell the file system "don't tell me this write is done until it
really is").

The OS' contribution is to help the drive decide what to "pin" in the
flash part (and perhaps help it to decide on partitioning between
long term storage and caching). If the right things are pinned,
booting and loading frequently-used apps can be fast. Meanwhile, if
the power dies, the short term cache is safe.

   --John

On Wed, 17 Oct 2007, Kevin van Haaren wrote:

> I believe the difference in boot times is far more than a few seconds,
> especially in Windows which typically takes longer than a Mac to boot.

Not that it's *quite* the same thing, but a few months ago I met a guy
that had a 4gb Firewire flash memory stick that had a bootable copy of
Tiger on it that he was using as a test-boot drive.

I'd been curious about these, as I figured that they ought to boot a lot
faster than hard drives (no spinning discs...), but held off on getting
one because they're so expensive (2-3x as much as a portable Firewire
drive, and with 20x less storage space).

I was surprised to learn that, in his experience, boot times from this
device weren't noticeably faster than they were from a regular internal
or external hard drive. The main advantage, as he saw it, was that being
solid state, it ought to be more rugged than a hard drive, but in day to
day usage, the performance wasn't really any different than HDs.

Hybrid drives are assumed to be faster & more reliable than traditional
hard drives for the much the same reasons that I'd been assuming that
the Firewire flash drive should have been faster. But is that speed
difference less impressive in reality than is being assumed? If so, is
the reliability gain enough to make the additional cost worth it?

Has anyone else tried a Firewire flash drive for booting, and if so, how
well did it work for you? I'm curious...

--
Chris Devers