Solid State Disk Drives

 

Solid State disks (SSD), (also called solid state drive) are devices that use exclusively semiconductor memory components to store digital data. The memory components are the same ones used for the different types of computer memory: work memory, cache memory and embedded memory.

 

Solid state disk (SSD) is a data storage device that uses memory chips, such as SDRAM‘s, to store data, instead of the spinning platters found in conventional hard disk drives. While not technically "disks" in any sense, these devices are so named because they are typically used as replacements for disk drives in situations where space, power supply, or ruggedness concerns would make conventional drives impractical. Solid state disks solve the problem of physical constraints by replacing hard disk drives with high speed circuitry. Instead of a rotating disk, a solid state disk uses memory chips (typically DDR RAM) to read and write data. Solid state disks allow storage to catch up with the rest of the computing world. Instead of allowing expensive servers to constantly sit and wait for hard disk drives your servers are busy increasing performance and operations per second. When servers are doing more transactions every day, the bottom line is directly improved. The immediate concern voiced about solid state disks regards data persistence and volatility.

 

Unlike magnetic disk drives, RAM-based disks require power to maintain their data. The solution to this is surprisingly simple: solid state disks includes backup batteries and backup hard disk drives so that any data written to the DDR RAM can be mirrored to or backed-up onto these drives.

 

The two primary advantages resulting from using solid state memory components instead of mechanical devices to store data are higher ruggedness and significantly improved performance. Performance improvements result from the very fast and predictable access times associated with the semiconductor memory components and from the drastic reduction of latency and seek times. Additional benefits are the extended operating temperature range and the lower power consumption.

 

Most solid state disks have a HDD-compatible form factor, systems interconnect and electric interconnect. This enables solid state disks to be added to the system very much like the magnetic or optical disk drives. From a system solution perspective, solid state disks can be regarded nowadays as HDD replacements. From a system architecture and functional perspective, they complement mechanical disk drives and have the capability to enhance the performance of individual mechanical drives and drive clusters.

The term "SSD" is used for two different kinds of products.

DRAM-based Solid State Disk (D-SSD)
SRAM-based Solid State Disk (S-SSD)
Flash memory-based Solid State Disk (F-SSD)

The first two (D-SSD and S-SSD) are based on volatile memory components and need a data retention mechanism when the power supply is removed. This can be either built-in batteries, or a non-volatile mechanic or semiconductor backup. F-SSDs use Flash memory chips that are non-volatile memory components. Some solid state disks may use a combination of memory components in order to improve certain specifications.

SRAMs have the fastest read and write cycle times. They do not need a periodic refresh cycle to preserve the stored contents. S-SSD drawbacks are the volatile nature of the memory cell and its larger size, which leads to a lower storage density and higher cost per stored bit.

DRAMs have fast read and write cycle times as well. However, they need to periodically refresh their contents, which downgrade the average transfer performance. The DRAM cell size is considerably smaller than that of SRAMs and the storage density accordingly higher. The DRAM cost per stored bit is currently the lowest.

Flash memory has a read access time that is comparable to that of DRAMs, however, the write cycle time is significantly longer. In addition, the write operation needs to be performed in conjunction with an erase operation, simultaneously for a group of locations, called page or sector. This complicates the write process and the associated circuitry. Flash memory cell size is smaller than that of DRAM and can be reduced at a faster pace. This creates the premise for lower storage cost. From a system perspective, the main advantage of F-SSD is the non-volatile nature of its storage cell, which puts it at par with magnetic and optical disk drives.

 

Solid State Disk Drive Producers

Samsung

Samsung releases the world’s first PCs embedded with a 32-Gigabyte (GB) NAND flash-based solid state disk (SSD). This mark the first time that NAND flash has moved into a commercial mobile computing application and is a breakthrough that will pave the way for replacing hard disk drives with NAND flash-based memory disks.

The Samsung Q1, an ultra-mobile computing device and the Q30, a 12.1-inch screen notebook PC, will be available in the Korean market from early June.

The two new SSD-enabled PC offerings are designed for optimal portability and resolve many of the traditional challenges of mobile computers. The data in flash memory are much more secure against external shocks that can occur when transporting a mobile computer. The SSD can withstand about twice the impact that would cripple a regular hard disk drive. In addition, stored data can be more easily retrieved from flash memory than traditional hard drives when PCs are dropped or liquid is spilled on the device. These mobile computing devices are the ideal solution for professionals and executives who are constantly on the move.

The SSD reads 300 percent faster (53MB/s) and writes 150 percent quicker (28MB/s) than normal hard drives. As a result, multiple application programs can operate simultaneously and large volumes of data can be edited and reproduced more efficiently.
The Microsoft Windows XP operating system will boot up 25-50% faster on the SSD than on other drives—good news for those in a hurry. Moreover, the typical 1.8-inch hard disk drive weighs around 50 grams; whereas the SSD is 20 to 30 grams lighter, depending on the package type. 

The typical notebook PC will generate around 30dB of operating noise, while the Q30-SSD will operate in complete silence. This is an unprecedented feature for people who want to use their PC in a library or other places where noise is not allowed.

The Q1-SSD will show video or still photos as well as play audio without having to be booted up first. This “instant on” feature provides access to multimedia content such as digital multimedia broadcasting (DMB) TV at least 30% faster than with a portable multimedia player (PMP).

DMB TV receivers are embedded in both PCs, which will bring extra enjoyment to users during this summer’s World Cup competition.
Msystems

mSSD solid state disks (formerly FFD) provide rugged, high-performance and highly reliable data storage. They withstand extreme temperatures, shock and vibration, even in harsh conditions, without compromising data integrity. mSSDs are drop-in replacements for SATA, IDE and SCSI mechanical hard disk drives, eliminating seek time, latency and potential failures inherent in conventional rotating magnetic media. mSSD reduces the total cost of ownership while enhancing reliability in the field.

Msystems patented TrueFFS (True Flash File System) technology, a de-facto standard in the industry, brings top data reliability and flash disk endurance to the mSSD product line.  It manages the flash and applies advanced error detection and correction algorithms to guarantee data reliability. It performs dynamic and static wear-leveling to extend the lifespan of raw flash material for mSSD products way beyond fab specifications.

TrueFFS enables these benefits:

·                     Data reliability: 99.999%

·                     MTBF: >1,400,000 hours (in the field)

·                     Embedded EDC/ECC: based on BCH algorithm

·                     Data integrity under power-cycling

·                     Bad block mapping

·                     Dynamic and static wear-leveling algorithms

·                     Endurance: >5,000,000 write/erase cycles, unlimited read cycles

·                     Warranty: 5 years

·                     mSSD takes rugged to new levels:

·                     Operating temperature: -40°C to +85°C

·                     Storage temperature: -55°C to +95°C

·                     Operating altitude: up to +80,000 feet

·                     Operating shock: 1,500G per MIL-STD-810F

·                     Operating vibration: 16.3G RMS per MIL-STD-810F, (random, 10-2000Hz, 3 vibrations axes)

·                     Humidity: 5% to 95% relative, non-condensing mSSD 4K (formerly IDE 4000) cost-effective solid state disk is available with:

·                     Form factor: 1.8", 2.5"

·                     Density: up to 8Gbytes

·                     Interface: IDE (PIO 0-4 and DMA 0-2)

·                     Low power consumption

·                     Case height: 7.4mm

·                     Warranty: 3 years

 

Building Solid State Disk Devices

The scope of this article will be limited to solid state disk devices made from flash memory. Flash memory is a solid state memory (no moving parts) that is non-volatile (the memory maintains data even after all power sources have been disconnected). Flash memory can withstand tremendous physical shock and is reasonably fast (the flash memory solutions covered in this article are slightly slower than a EIDE hard disk for write operations, and much faster for read operations). One very important aspect of flash memory, the ramifications of which will be discussed later in this article, is that each sector has a limited rewrite capacity. You can only write, erase, and write again to a sector of flash memory a certain number of times before the sector becomes permanently unusable. Although many flash memory products automatically map bad blocks, and although some even distribute write operations evenly throughout the unit, the fact remains that there exists a limit to the amount of writing that can be done to the device. Competitive units have between 1,000,000 and 10,000,000 writes per sector in their specification. This figure varies due to the temperature of the environment.

Specifically, we will be discussing ATA compatible compact-flash units and the M-Systems DiskOnChip® flash memory unit. ATA compatible compact-flash cards are quite popular as storage media for digital cameras. Of particular interest is the fact that they pin out directly to the IDE bus and are compatible with the ATA command set. Therefore, with a very simple and low-cost adaptor, these devices can be attached directly to an IDE bus in a computer. Once implemented in this manner, operating systems such as FreeBSD see the device as a normal hard disk (albeit small). The M-Systems DiskOnChip product is based on the same underlying flash memory technology as ATA compatible compact-flash cards, but resides in a DIP form factor and is not ATA compatible. To use such a device, not only must you install it on a motherboard that has a DiskOnChip socket, you must also build the `fla` driver into any FreeBSD kernel you wish to use it with. Further, there is critical, manufacturer-specific data residing in the boot sector of this device, so you must take care not to install the FreeBSD (or any other) boot loader when using this.

A few kernel options are of specific interest to those creating an embedded FreeBSD system.

First, all embedded FreeBSD systems that use flash memory as system disk will be interested in memory disks and memory file systems. Because of the limited number of writes that can be done to flash memory, the disk and the file systems on the disk will most likely be mounted read-only. In this environment, file systems such as /tmp and /var are mounted as memory file systems to allow the system to create logs and update counters and temporary files. Memory file systems are a critical component to a successful solid state FreeBSD implementation.

You should make sure the following lines exist in your kernel configuration file:

options         MFS             # Memory Filesystem

options         MD_ROOT         # md device usable as a potential root device

pseudo-device   md              # memory disk

Second, if you will be using the M-Systems DiskOnChip® product, you must also include this line:

device          fla0    at isa?

The post-boot initialization of an embedded FreeBSD system is controlled by /etc/rc.diskless2 (/etc/rc.diskless1 is for BOOTP diskless boot). This initialization script is invoked by placing a line in /etc/rc.conf as follows:

diskless_mount=/etc/rc.diskless2

rc.diskless2 mounts /var as a memory filesystem, makes a configurable list of directories in /var with the mkdir(1) command, changes modes on some of those directories, and extracts a list of device entries to copy to a writable (again, a memory filesystem) /dev partition. In the execution of /etc/rc.diskless2, one other rc.conf variable comes into play - varsize. The /etc/rc.diskless2 file creates a /var partition based on the value of this variable in rc.conf:

varsize=8192

Remember that this value is in sectors. The creation of the /dev partition by /etc/rc.diskless2, however, is governed by a hard-coded value of 4096 sectors. It is trivial to change this entry in the /etc/rc.diskless2 file itself, although you should not need more space than that for /dev.

It is important to remember that the /etc/rc.diskless2 script assumes that you have already removed your conventional /tmp partition and replaced it with a symbolic link to /var/tmp. Because tmp is one of the directories created in /var by the /etc/rc.diskless2 script, and because /var is a memory filesystem (which is mounted read-write), /tmp will now be a directory that is read-write as well.

The fact that /var and /dev are read-write filesystems is an important distinction, as the / partition (and any other partitions you may have on your flash media) should be mounted read-only. Remember that in Section 1 we detailed the limitations of flash memory - specifically the limited write capability. The importance of not mounting filesystems on flash media read-write, and the importance of not using a swap file, cannot be overstated. A swap file on a busy system can burn through a piece of flash media in less than one year. Heavy logging or temporary file creation and destruction can do the same. Therefore, in addition to removing the swap and /proc entries from your /etc/fstab file, you should also change the Options field for each filesystem to ro as follows:

# Device                Mountpoint      FStype  Options         Dump    Pass#

/dev/ad0s1a             /               ufs     ro              1       1

A few applications in the average system will immediately begin to fail as a result of this change. For instance, ports will not install from the ports tree because the /var/db/port.mkversion file does not exist. cron will not run properly as a result of missing cron tabs in the /var created by /etc/rc.diskless2, and syslog and dhcp will encounter problems as well as a result of the read-only filesystem and missing items in the /var that /etc/rc.diskless2 has created. These are only temporary problems though, and are addressed, along with solutions to the execution of other common software packages in Section 6.

An important thing to remember is that a filesystem that was mounted read-only with /etc/fstab can be made read-write at any time by issuing the command:

# /sbin/mount -uw partition

and can be toggled back to read-only with the command:

# /sbin/mount -ur partition