Anatomy of SSDs

SSDs are perhaps the hottest new hardware development in storage. They offer the promise of very high performance and low power. From the lowly laptop SSD to the ultra high-performance of Fusion-IO and Texas Memory, SSDs have a great deal of buzz about them as witnessed by the number of reviews and tech-focused articles around the web and the print media. As with all technologies there are benefits to them and there are limitations. This goal of this article is to help understand the technology including the benefits and limitations by beginning with the building blocks the NAND Flash chips. To truly understand them you have to start with the underlying technology, floating-gate transistors.

Floating-Gate Transistors

The concept of a floating-gate transistor is the key to understanding Flash memory or Flash storage and thus SSDs. Figure 1 below from Anandtech illustrates the floating-gate transistor

Figure 1 – Floating Gate Transistor

Between the floating gate and the substrate is the tunnel oxide. This is the barrier to the floating gate and is through which the electrons “tunnel” into the floating gate. The transistor either has electrons tunneled into the floating gate (indicating a logical 0) or does not have any electrons tunneled into the floating gate (indicating a logical 1). The process of forcing electrons to or from the floating gate, called Fowler-Nordheim Tunneling (F-N Tunneling), is achieved by applying a voltage between the control gate and the source or drain. When the charge is removed the floating gate either retains the electrons if they were tunneled into the gate, or has no extra electrons if they were removed. This allows Flash memory to retain values after power is removed.

To program (write) to the transistor, which creates a logical 0, a positive voltage is applied to the drain which activates the electrons underneath the floating gate in the substrate. Then a larger positive voltage is applied to the control gate forcing the electrons to tunnel into the floating gate.

To erase the transistor, a negative voltage is applied to the control gate and a positive voltage is applied to the source. This forces the electrons out of the floating gate and into the source.

To read the transistor a positive voltage that is lower than the write voltage is applied to the control gate. An additional positive voltage, also lower than the write voltage, is applied to the drain. The current between the source and the drain will determine if the floating gate has extra electrons (logical 0) or does not (logical 1).

It does take much more time to write (program) or erase the floating gate because of the time for the electrons to tunnel into the floating gate in the case of a write (program). Erasing can also take more time because the electrons have to move out of the floating gate and into the source. This erase process takes slightly less voltage than the write operation but much more than the read operation.

Currently there are two types of floating-gate arrays (cells) in use. The Single Level Cell (SLC) is read using the previously mentioned technique but the current is just sensed as being present or not (i.e. it’s not actually measured). In the case of Multi-Level Cells (MLC) two levels of current can be sensed allowing the transistor to store two bits of if information instead of one.

The floating-gate transistors serve as the basis of what are called NAND Flash chips. In the next section how flash chips are constructed from floating-gate transistors will be discussed.

Creating NAND Flash Units

The most common cell used in SSDs is the NAND Flash. For this device, the transistors are connected in series. Then these groups are connected in a NOR style where each line is connected directly to ground and the other is connected to a bit line. This arrangement has advantages for cost, density, and power as well as performance but it is a compromise that has some implications as will be discussed later in the article.

In NAND Flash memory the cells are first arranged into pages. Typically, a page is 4KB in size. Then the pages are combined to form a block. A block, illustrated below in Figure 2, is typically formed from 128 pages giving a block a size of 512KB.

Figure 2 – Block View of NAND Flash

The blocks are combined into a plane. Typically a total of 1,024 blocks are combined into a plane, giving it a typical size of 512MB as show in Figure 3.

Figure 3 – Plane View of NAND Flash

Typically there will be multiple planes in a single NAND-flash die. Manufacturers will also put several dies into a single NAND flash chip. Then you can put multiple chips in a single drive.

Discussion of Features/Limitations of NAND Flash

Before you use a new technology it is always a good idea to have the best possible understanding of these so that you can make better, informed decisions and have a basis for understanding test results (good and bad). SSDs are an exciting new technology that people are using in everyday life ranging from simple USB drives (not really SSD but they still use NAND Flash), to laptop drives, and even to enterprise class SSDs. Much of this use is based primarily on performance considerations but also on power and the fact that it has no moving parts (think laptops). In the following sub-sections, these attributes as well as others will be discussed from a feature/limitation perspective. But, keep in mind that the features/limitations of NAND Flash are being discussed, not the features/limitations of SSD drives themselves.

Performance (Asymmetric and Otherwise)

Performance is probably the primary reason for the excitement around SSDs (as I always say, who doesn’t like more performance?). To better understand performance of the NAND Flash chips, recall that there are two types of cells: SLC and MLC. SLC can storage a single bit of data while MLC can store two bits. MLC sounds really great because you can store twice the amount of data compared to SLC but you pay a penalty for this extra data density. Table 1, with data from this link illustrates the differences between SLC and MLC from a performance perspective.

Table 1 – Performance Timings of SLC and MLC

Notice that the read performance of MLC is twice as slow as SLC as could be expected, but the write performance is over 3 times slower.

Writing to NAND Flash is a multi-step process. In general, for writing to cells that have existing data, you first have to read the cells, followed by erasing the cells, and then program (write) to the cells. If you look in Table 1, the read step is fairly fast, but the erase step is two orders of magnitude slower than reading. The programming step, while not as slow as erasing, is still 10-20 times slower than a read. Consequently, writing is not a fast operation compared to read.

This data points out that NAND Flash provides asymmetric performance. Reads are amazingly fast. Programs (writes) are about 10 times slower than reads. Erase/Program (writing over existing data) is 2-3 orders of magnitude slower than reads.

Data Retention Time

One of the attractive features of NAND Flash (SSD) is that they retain their information after the power is removed. It is reported that the data can be retained for 10-100 years. The reason that they don’t last longer is that over time the electrons can “leak” from the floating gate resulting in data corruption (i.e. “bit rot”). In addition, as the number of erase/program cycles increases, the retention period shortens (see this link for more information).

Data Corruption Due to Die Shrink

An additional problem pointed out in this link is that as the cells shrink in size, the probability of causing data corruption increases. Recall that you need fairly high voltages to erase/program cells – in some cases, this can be up to 12V. As the cell shrink, the distance between the source and drain diminishes, but the voltages stay approximately the same. So the probability that a erase/program step might “disturb” a neighboring cell, possibly causing data corruption, increases. Getting to larger densities may not be easily achieved because of this possible data corruption with this link indicating that the lower limit may be 20 nm. However, companies are actively researching new materials and techniques to reduce the required voltages, allowing higher densities with the same data corruption probability.

Erase/Write Limits

This is probably one of the most mentioned limitations of NAND Flash chips – the limit to the number of times the transistor can go through an erase and write (program) cycle. Recall that to write data, except for never been written-to cells, the data must be first read, then certain cells must be erased and then certain cells must be programmed (written). After a certain number of cycles, the transistor can no longer retain electrons in the floating gate to a level that allows it to be used for storing data. This limit is commonly referred to as the erase-write limit or just as the rewrite limit.

From the SNIA paper previously referenced, The following are the typical erase/program cycle limits for SLC and MLC NAND based flash cells:

• SLC: 100,000
  
• MLC: 5,000-10,000
  

These are typical erase/write cycle limits but the exact number depends upon the manufacturer. For example in late 2008, Micron and Sun announced a SLC with a limit of 1,000,000 cycles.

Block Erasing

NAND Flash cells can be very easily read one byte (bit) at a time. Lower voltages are applied to the floating-gate transistor and the resulting current is measured. You can easily write to a single page if it is pristine (i.e. no data has been written to it before). However, if there is existing data then the data needs to be erased as part of the write cycle. Current NAND Flash chips can only be erased in units of blocks (512KB).

The block erase limitation can have a very large impact on performance. For example, if an application is re-writing data, then it is possible that only a few bytes in a block will need to be erased as part of the re-write. But, this forces the entire contents of the block (512KB) to be read, temporarily stored somewhere, the block is erased, the existing non-changing data is merged with the new data and the resulting block is written to the block. To possibly change a few bytes, the entire 512KB of data in the block has to be erased. This also includes the case when the block is partially used – a rewrite of any data in the block will force the entire block to be erased even if most of the block is not holding any data.

What is potentially important of this limitation as well is that all of the cells in the block have to be erased, and possibly written, consuming an erase/program cycle of that cell. Recall that NAND Flash have a limited number of erase/program cycles, so using those cycles because only a very small part of the block is updated is a very expensive operation. In addition, the erase/program cycle is much slower than the read operation, possibly reducing write throughput.

Seek Times

One advantage that NAND Flash has over rotating media is seek time. For rotational media, the location of the data has to be computed, the drive head has to be moved to the right location and there may be a pause for the disk to rotate to the correct spot. For data that is spread over the disk, this may force the drive head to move all over the disk possibly greatly reducing throughput. However, SSD drives constructed from NAND Flash cells don’t suffer from this problem.

For NAND Flash, only the location of the bits/bytes needs to be computed and then the read operation can take place. There is no mechanical movement – it is all done electrically . Consequently the seek time, the amount of time it takes to find the data, is greatly reduced. For workloads where seek times are important (e.g. IOPS driven workloads), SSDs have a huge performance advantage over hard drives. In fact, you can do reads in parallel if the drive controller and the drive is capable of parallel operations.

Summary

In summary, the features/limitations of NAND Flash are:

• Very fast read performance
  
• Asymmetric read/write performance (reads are 2-3 orders of magnitude faster than writes)
  
• There are data retention limitations due to leakage and due to exercising the cells (i.e. using the erase/program cycles)
  
• Shrinking the dies to increase density increases the probability of data corruption from erase/program functions disturbing neighboring cells
  
• NAND Flash cells have a limited number of erase/program cycles before they can no longer retain data
  
• NAND Flash cells can read a byte at a time or read/write a page at a time, but an entire block must be erased if one cell in the block is erased
  
• Seek times for NAND Flash chips is much lower than hard drives
  

It may seem that the picture isn’t as rosy as reports have stated, but remember that these are the features/limitations of the NAND Flash cells themselves. The next section will discuss several techniques that manufacturers have employed to build SSD drives and help overcome or at least moderate some of the limitations of the drives, resulting in some very high performance drives.

Chasing the Devil in the Details