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First time i read some information about SSD's, i was surprised to learn they internally use NAND flash chips. This kind of memory is generally slow (low bandwidth) and have high latency while SSD's are just the opposite.

But here is how it works : SSD drives increase their bandwidth by using several NAND flash chips in parallel. In other words, they do some data striping (aka RAID0) across several chips (done by the controller).

What i don't understand is how SSD's drives have such a low latency, whereas they are using NAND chips? (or at least lot better than what a typical single NAND chip would do)


EDIT: I under-estimated NAND chip capabilities. USB drives, while powered by NAND's are mostly limited by USB protocol (which have a pretty high latency) and the USB controller. That explain their poor performance in some cases (as prunge explained).

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Where have you gotten the impression that the latency of NAND flash is so high? –  Mr Alpha Jun 27 '12 at 21:29
    
browsing files on a USB key or SD card is typically slower than same operation on a HDD. Wikipedia article on SSD also admit NAND chips are slow : "a single NAND chip is relatively slow, due to narrow (8/16 bit) asynchronous IO interface, and additional high latency of basic I/O operations". –  tigrou Jun 27 '12 at 21:45

2 Answers 2

up vote 6 down vote accepted

Flash chips have lower latency than magnetic disks, there are no moving parts so there is no need to wait for the disk to spin and heads move into position on the disk to read/write data. The lower latency of flash drives vs magnetic disks is the whole reason ReadyBoost works - which caches commonly used files on flash disks to reduce access times.

Flash disks have lower throughput than hard disks/SSDs because of the limited-bandwidth USB interface (SATA is faster than USB2) and cheaper hardware on the flash disks (no need to provide fast transfer speeds if USB will limit you anyway).

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Let's not forget USB3 though. Newer flash drives supporting SATA3 are giving >50MB/sec sequential (I have one, don't know about random read/write though). –  Drizzt321 Jun 28 '12 at 17:03

Reduced latency compared to what? Compared to mechanically spinning drives? No brainer on that one, no wait for the disk to spin to the right location, no need for the drive heads to move to the right spot, etc. If you're talking about total data transfer, then it spreads out the reads & writes across multiple NAND dies (sometimes multiple dies in the same physical chip package!), plus it can do things like pre-fetch into cache, and some controllers (Sandforce) do things like on the fly compression.

In terms of compared to other forms of flash memory? No clue, I haven't researched any of it.

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