Monday Jul 28, 2008

NAND Flash based SSDs

NAND-DIE.png

What is Flash?

Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed. The Flash technology is primarly used in memory cards and USB flash drives for general storage and transfer of data between coputers and other digital products. It is a specific type of EEPROM (Electrically Erasable Programmable Read-Only Memory) that is erased and (re)programmed in large blocks. In the early flash products, the entire chip had to be erased at once. Flash memory costs far less than byte-programmable EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile storage is needed.

Flash memory needs no power to maintain the information that is stored on the chip. In addition, flash memory offers fast read access times and better shock resistance then hard disks. Flash is able to withstand intense pressure, extremes of temparature and even immersion in water.

What is NAND?

The NAND flash architecture was introduced in 1989. These memories are accessed much like block devices such as hard disks or memory cards. Each block consists of a number of pages. The pages are typically 512, 2024 or 4096 bytes in size. Associated with each page are a few bytes (typically 12-16 bytes) that should be used for storage of an error detection and correction checksum.

While programming is performed on a page basis, erasure can only be performed on a block basis. Another limitation of NAND flash is data in a block can only be written sequentially. Number of operations (NOPs) is the number of times the sectors can be programmed. So far this number for MLC (Multy Level Cell) flash is always one whereas for SLC (Single Level Cell) flash it is 4.

NAND devices also require bad block management by a separate controller chip. SD cards, for example, include controller circuitry to perform bad block management and wear leveling.

SLC vs MLC NAND Flash

MLC (Multi Level Cell) NAND flash allows each memory cell to store 2 bits of information, compared to 1 bit-per-cell for SLC NAND flash, resulting in a larger capacity and lower bit cost. As a rule of thumb, MLC devices are available at twice the density of SLC devices of the same flash technology. Mature and proven, MLC technology is generally used in cost-sensitive consumer products such as cell phones and memory cards.

A significant portion of the NAND flash-based memory cards on the market today are made from MLC NAND, and the continuing rapid growth of this market can be considered an indication that the performance is meeting consumers' needs. Although the use of MLC technology offers the highest density (and the lowest cost), the tradeoff compared to single-bit-per cell is lower performance in the form of slower write (and potentially erase) speeds, as well as reduced write/erase cycling endurance.

Also, because of the storage of 2 bits per cell, the probability of bit error is higher than for SLC technology. However, this is partially compensated for by using error detection and correction codes (EDC). System designers have long been aware of the benefits of using EDC to detect and correct errors in systems using Hamming codes (common in memory subsystems) and Reed Solomon codes (common in hard drives and CD-ROMs).

SLC NAND is generally specified at 100,000 write/erase cycles per block with 1-bit ECC. MLC is generally specified at 10,000 cycles with ECC. While the datasheet for the MLC device does not specify the level of ECC required, the MLC manufacturers recommend 4-bit ECC when using this technology. Therefore, when using the same controller, a storage device using SLC will have an endurance value roughly 10 times that of a similar MLC-based product.

The following table shows the advantages and disadvantages of SLC Flash and MLC Flash:
SLC_vs_MLC.png
When we talk about Enterprise Class Flash Storage, we cleary talk about SLC based NAND Solid State Disks!

Why Solid Stated Disks?

One of the biggest advantages of Flash Based SSDs is their latency. The performance of Flash is a bit unusual as it is highly asymmetric. A block of flash must be erased before it can be written, which takes on the order of 1-2ms for a block. Writing to an erased flash requires around 200-300us. Most of the flash based disks try to maintain a pool of prefiously erased blocks, so that the latency of a write is just that of teh program operation. Read operations are much faster, 25-30us for 4k. Flash based SSDs also use internal DRAM Memory to assure a good write performance. The RAM is protected with a capacitor to avoid data losses on power outages. A capacitor doesn't require any maintenance.

Conventional storage solutions mix dynamic memory (DRAM) and hard drives; flash is interesting because it falls in a sweet spot between those two components for both cost and performance in that flash is significantly cheaper and denser than DRAM and also significantly faster than disk. Flash accordingly can augment the system to form a new tier in the storage hierarchy – perhaps the most significant new tier since the introduction of the disk drive with RAMAC in 1956.

ZFS by Sun Microsystems has been optimized to manage Flash SSD systems, both as cache as well as main storage facilities, available for OpenSolaris, Mac OS X, and the Linux operating system.

Business Cases for Flash Based SSDs

Bellow you can find a few business cases, where flash clearly is usefull and probably the technology of choice in the future:
  • As a LOG device for Databases, like Oracle Redo-Logs
  • As an extended, huge and solid cache for ZFS
  • As a ZIL (ZFS Intend Log) device of ZFS, which is similar to Redo-Logs in Oracle
  • As a MetaData device for SAM-QFS. QFS can separate the MetaData from real data. This improves the common file system operations like ls, find, by factors. It also improves the "Directory Name Lookup Cache" by factors on huge file systems (above 10Mio Files) as the traditional RAM wont be sufficient.
  • As a storage device for immense data transactions or databases with hughe amount of transactions per second; mostly WRITE as for read you can also use the RAM of the server
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