SSDs

SSDs are great. They have come down enormously in price over the past 10 years, and every evidence suggests that they will continue to do so. They have the ability to offer access times several orders of magnitude lower than rotating disks and consume less power.

One important concept to understand about SSDs is that although their read and write latencies are typically measured in 10's of microseconds, to overwrite an existing data in a flash block, it requires the entire flash block to be erased before the write can happen. A typical flash block size in SSD may be 128 KB, and even a 4 KB write I/O would require the entire block to be read, erased and then the existing data and new I/O to be finally written. The erase operation can take several milliseconds and without clever routines in the SSD firmware, would make writes painfully slow. To get around this limitation, SSDs are equipped with a RAM buffer, so they can acknowledge writes instantly, whereas the firmware internally moves data around flash blocks to optimize the overwrite process and wear leveling. However, the RAM buffer is volatile memory and would normally result in the possibility of data loss and corruption in the event of sudden power loss. To protect against this, SSDs can have power loss protection, which is accomplished by having a large capacitor on board, to store enough power to flush any outstanding writes to flash.

One of the biggest trends in recent years is the different tiers of SSDs that have become available. Broadly speaking, these can be broken down into the following categories.