Mainstream solid-state drives
Learn about the differences between enterprise solid-state drives (SSDs) and mainstream SSDs (previously called read intensive SSDs).
Traditionally, enterprise SSDs are built on high-endurance multi-level cell (MLC) flash and can handle up to 10 drive writes per day. These SSDs are referred to as enterprise SSDs in this document (previously called eMLC). Now, because of software advances and industry demands, less write intensive SSDs can be used with applications where write operations are less frequent. IBM® offers several 4K mainstream SSDs, including feature codes (FCs) ES8Y, ES8Z, ES96, ES97, ESE7, ESE8, ES83, ES84, ES92, ES93, ESE1, and ESE2.
Differences between mainstream and enterprise SSDs
Mainstream SSDs are less expensive to use, but also have lower endurance and random write performance.
The NAND flash used in mainstream drives tend to be of lower endurance than the NAND flash used in SSDs that are targeted for the more write intensive workloads. Therefore, the number of write operations to a mainstream drive is limited (typically, one drive write per day (DWPD) versus 10 DWPD on an enterprise drive).
A drive write per day writes the entire capacity of the drive in 24 hours. For example, a DWPD for a 387 GB drive writes 387 GB of data on the drive in 24 hours. You can write more data in a day, but the DWPD is the average usage rate by which the life span of the drive is calculated. Since many applications only require about 1 DWPD, these are the most commonly used drives in the industry and thus used for mainstream applications. Only applications that require high endurance or the highest random write performance need enterprise drives.
SSDs have more NAND flash capacity than the rated user capacity of the drive. This extra capacity, called over-provisioning, is used by the SSD controller during the operation of the drive. When more over-provisioning is available, the controller extends the life of the flash more effectively. NAND flash can be written (programmed) and read in small units that are called pages, individually, but to rewrite that page, the page must first be erased and then programmed again.
Due to the architecture of NAND flash, erase operations are completed on the block level and not the page level. Each block contains hundreds to thousands of pages. Therefore, to erase a block, all of the valid data must first be transferred to another block and then you can erase the block. The SSD controller then searches for blocks with large proportions of pages that have data that can be erased. Then, the SSD controller moves and combines the pages of data that must be retained to previously erased blocks, which then frees these new blocks for erasing.
This process of moving data to free up blocks for erasing is called garbage collection. Increasing the over-provisioning of an SSD enables the controller to be more efficient in garbage collection and minimizes extra read and program operations.
All of these background operations result in more data that is written to the flash than what is written to the drive. The ratio of data that is written to flash and the data that is written to the drive is referred to as the write amplification. All else being equal, write amplification is greater for drives with lower over-provisioning.
The cost per GB of a mainstream drive is typically less than the cost per GB of an enterprise drive. The cost is low because the amount of over-provisioning is less and almost all the flash memory in the drive is available for storing data.
Mainstream drives are similar to enterprise drives in terms of read performance. However, because of the lower over-provisioning of the mainstream drives, the random write performance is reduced because of the higher number of background operations that are needed for garbage collection and associated write amplification. Thus, lower over-provisioning decreases both performance and endurance. Read performance is not impacted.
Endurance differences imply that when you form disk arrays, you must not mix mainstream drives with enterprise drives because the PCIe SAS adapter stripes data across drives, thus sending equal data to each drive. The IBM PCIe SAS adapters do not allow mixing of mainstream drives with enterprise drives when you create RAID arrays.
You must monitor end of life symptoms for mainstream drives because of their endurance limitations. Internally, when the drive nears the end of life, a predictive failure analysis (PFA) trip is generated and an operating system message is logged. When this trip is generated, the drive continues to run but it must be replaced as soon as possible. The PFA trip code for end of life is the same as the PFA trip code for thermal failures. Therefore, you can determine the root cause of a failure by using operating system support that is provided by a fuel gauge command.
A mainstream drive is not suitable for write intensive workloads. Assuming a typical heavily random workload, at about 3394 TB of write operations to a 1.9 TB mainstream drive, the drive is at its maximum projected write capability. If the write operations exceed the drive's maximum write capacity, the write operation takes more time to complete. A predictive failure analysis (PFA) message indicates that you should replace the drive.
If you ignore the PFA message and if you continue to send the write operation requests to the drive, the drive is unable to accept write commands and only accepts read commands for some time. A failed write operation results in a more serious error message that indicates that the drive must be replaced.
The nature of the workload impacts the maximum write operation capacity. For example, if a high percentage of sequentially oriented write operations is used instead of random oriented write operations, the maximum write operation capacity increases. You must periodically check the percentage of the drive's remaining write life, and if necessary, adjust the workload or reassign the drive. Check the remaining life on each of the mainstream drives individually, even if all of the drives are in the same array.
If a mainstream drive reaches its maximum write operation capability during the warranty period, IBM replaces the drive at no cost. The warranty period of the drive is defined by the server type under which the drive feature code is ordered and it is either 3 years or 1 year for IBM Power Systems™ processor-based servers. After the warranty period, the replacement of the drive is not covered under IBM maintenance if the maximum number of write operations exceed the threshold value. You must order a new, chargeable SSD as a replacement. Other aspects of SSD maintenance are consistent with SSDs that are not mainstream drives.
For instructions about using the fuel gauge command, select the option for the operating system you are using:
