At 8:46 PM +0200 10/26/2010, Mac User #330250 wrote:
> Not the OS. The "cache hit" tracking in the device's firmware. It
doesn't know files, just block addresses.
Well, I read differently. This was discussed when Windows 7 was introduced.
Apparently it can make use of this functionality and advice the firmware to
hold certain "importaint" files, err, data in in the SDD part of the hybrid
HDD.
What I read is that the operating system can make heavy use of this hybrid
thing, and if it does, it can be very very useful.
No. There is no special knowledge being exchanged between the OS and
the hybrid drive. From the OS' POV the drive is simply a normal hard
drive.
In fact, many of the hybrid drives are even more limited than I've
represented in this thread. The ideal case, as I described, is one
where the flash memory is used to hold data bi-directionally. That
means it speeds reads of data already contained therein. And it
speeds writes by accepting the data and telling the OS the operation
is done -- before the write-through to the HD is completed. But in
fact, that doesn't seem to be the case with most of the hybrids on
the market today! Turns out that most are simply "read" caches with
no write-through capability.
Case in point: the Seagate Momentus XT. Outside, this is a (up to)
500 GB laptop drive, with a 3 Gbps SATA or USB 3 interface. Inside,
it has a 32 MB cache and a 4 GB SLC NAND flash bank (SSD), as well as
the normal 7200 rpm HD mechanism. The NAND block is ONLY used as a
*read* cache. That means that ALL writes to the drive hit the main
cache only, then go directly to the HD. The NAND isn't filled until
you pull off the HD repeatedly, +/- the caching algorithm!
Why do it that way? Well, it reduces the number of writes to the
flash memory by quite a bit. This gives the brick a longer life.
... Remember, the goal is to make this thing work at least one day
beyond the warranty period. (Seagate is doing a 5 yr warranty on
them, btw).
- Dan.
--
- Psychoceramic Emeritus; South Jersey, USA, Earth.
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