четверг, 20 сентября 2012 г.

типы SSD


Сейчас есть 3 основных типа:

3. What are the types of SSDs?
Solid state drives (SSDs) based on flash memory generally demonstrate lower latencies than the hard disk drives (HDDs), often enabling faster response times. For random read workloads, SSDs  deliver higher throughput relative to HDDs.
a. Based on Nand Flash
I. SLC, or Single Level Cell, allows for the storage of one bit of information per NAND memory cell. SLC NAND offers relatively fast read and write capabilities, high endurance, and relatively simple error correction algorithms. SLC  is  typically  the most expensive NAND technology.  With SLC drives each cell is spec’d to last for around 100K writes. Reads are unlimited.  SLC drives are more suited for enterprise environments because of their durability.  They can be cost prohibitive in consumer applications.
II. MLC, or Multi Level Cell, technology in general is less robust than SLC as there are two bits stored in each cell. If one cell is lost two bits will be lost.   With MLC drives each cell is spec’d to last between 3,000 to 5,000 writes. The drives are usually available in larger capacities and are usually less expensive. MLC based SSDs are being used in enterprise applications deploying smart management techniques such as overprovisioning and endurance management (defined later in document).
III. eMLC, or enterprise MLC is a variant of MLC technology that is harvested from the highest quality portion of the NAND wafer and programmed uniquely to increase erase cycles.  eMLC achieves endurance levels of 30,000 write cycles, whereas some of the newest MLC  only has 3,000 write cycles. eMLC makes a tradeoff to enable this endurance by giving up  data retention.  eMLC addresses that problem by lengthening the flash memory chips’ internal page programming (tProg) cycle which creates a better, more lasting data write, but slows write performance. Since eMLC SSDs are somewhere between MLC and SLC on write endurance, their price is usually between the two types.  By adding advanced  endurance management techniques, this technology  can be successfully used in general purpose enterprise applications.

b. Based on Host Interface
I. SATA SSD: SATA SSDs are based on the  industry standard SATA interface. SATA SSDs provide reasonable performance for enterprise servers.
II. SAS SSD: SAS SSDs are based on the industry standard SAS interface. SAS SSDs combine superior reliability, data integrity,  and  data fail recovery  making them  suitable for enterprise applications.


Endurance, NAND P/E Cycle: 100K SLC, 30K eMLC, 3K MLC

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