Article: AMD's Mobile Strategy
By: Seni (seniike.delete@this.hotmail.com), December 20, 2011 6:24 pm
Room: Moderated Discussions
EduardoS (no@spam.com) on 12/20/11 wrote:
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>Seni (seniike@hotmail.com) on 12/20/11 wrote:
>---------------------------
>>Actually, the economics of small NAND caches are even worse than for small SSDs,
>>since you end up paying $40 for a minimal hard drive, then another $40 for a minimal
>>SSD, and then the SSD part turns out be slow because it doesn't have enough chips,
>>and NAND performance scales with chip count.
>
>If a flash cache is included in the HD it will increase it's price by $10, maybe
>$20, there is no need for many chips, just one or two, streamming accesses bypass
>the cache and goes to rotating media, random accesses goes to the flash, wich is
>way faster than rotating media for this access pattern, also, by skiping streaming
>accesses it saves spaces on the flash memory...
>
>Well, but you are right that it doesn't make the drive cheaper, but it solves the
>performance problem and may reduce significantly the enry cost of flash storage
>wich is about $100 today for anything usefull.
This already exists, but hybrid drives are neither as cheap, nor as fast as you expect. The Momentus XT, for example, is costing like $100 more than a normal drive, and performs more like a slightly faster normal drive, but is nowhere close to the speed of the SSDs.
It's still a decent option for laptops that only have one drive bay and yet need more than 250GB. But the fundamental performance and cost issues mean it can't really be done the way you'd like.
If you only have 1-2 NAND chips to keep it cheap, the cache miss rate will be too high and peformance will be mostly HD-like. If you have a 4-8 NAND chips to keep cache hitrate up, the price goes up and it's no longer cheap enough to compete with a pure SSD approach. The $40 you spent on mechanical drive components is $40 you could have spent adding 40GB to your SSD capacity if you ditched the mechanical side.
And with a hybrid drive, not only do you have to pay extra on for the NAND chips, you also have to pay extra for the flash-facing side of the controller, which now needs its own cores and high-pincount bus. So there's an unavoidable floor cost there too. It's a bad deal to pay both floor costs instead of one.
You just can't win, except maybe in a very narrow capacity range.
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>Seni (seniike@hotmail.com) on 12/20/11 wrote:
>---------------------------
>>Actually, the economics of small NAND caches are even worse than for small SSDs,
>>since you end up paying $40 for a minimal hard drive, then another $40 for a minimal
>>SSD, and then the SSD part turns out be slow because it doesn't have enough chips,
>>and NAND performance scales with chip count.
>
>If a flash cache is included in the HD it will increase it's price by $10, maybe
>$20, there is no need for many chips, just one or two, streamming accesses bypass
>the cache and goes to rotating media, random accesses goes to the flash, wich is
>way faster than rotating media for this access pattern, also, by skiping streaming
>accesses it saves spaces on the flash memory...
>
>Well, but you are right that it doesn't make the drive cheaper, but it solves the
>performance problem and may reduce significantly the enry cost of flash storage
>wich is about $100 today for anything usefull.
This already exists, but hybrid drives are neither as cheap, nor as fast as you expect. The Momentus XT, for example, is costing like $100 more than a normal drive, and performs more like a slightly faster normal drive, but is nowhere close to the speed of the SSDs.
It's still a decent option for laptops that only have one drive bay and yet need more than 250GB. But the fundamental performance and cost issues mean it can't really be done the way you'd like.
If you only have 1-2 NAND chips to keep it cheap, the cache miss rate will be too high and peformance will be mostly HD-like. If you have a 4-8 NAND chips to keep cache hitrate up, the price goes up and it's no longer cheap enough to compete with a pure SSD approach. The $40 you spent on mechanical drive components is $40 you could have spent adding 40GB to your SSD capacity if you ditched the mechanical side.
And with a hybrid drive, not only do you have to pay extra on for the NAND chips, you also have to pay extra for the flash-facing side of the controller, which now needs its own cores and high-pincount bus. So there's an unavoidable floor cost there too. It's a bad deal to pay both floor costs instead of one.
You just can't win, except maybe in a very narrow capacity range.
