Administrator Guide

Gauging the percentage of life remaining for SSDs
An SSD can be written and erased a limited number of times. Through the SSD Life Left disk property, you can gauge the percentage of
disk life remaining. This value is polled every 5 minutes. When the value decreases to 20%, an event is logged with Informational severity.
This event is logged again with Warning severity when the value decreases to 5%, 2% or 1%, and 0%. If a disk crosses more than one
percentage threshold during a polling period, only the lowest percentage will be reported. When the value decreases to 0%, the integrity
of the data is not guaranteed. To prevent data integrity issues, replace the SSD when the value decreases to 5% of life remaining.
You can view the value of the SSD Life Left property through the Disk Information panel. In the front view of the enclosure in the System
topic, hover the cursor over any disk to view its properties. You can also view the Disk Information panel through the Pools topic. Select
the pool for the disk group in the pools table, select the disk group in the Related Disk Groups table, and then hover the cursor over the
disk in the Related Disks table.
All-flash array
The all-flash array feature, enabled by default, allows systems to run exclusively with disk groups that consist of SSDs, providing the ability
to have a homogeneous SSD-only configuration. Systems using an all-flash array have one tier that consists solely of SSDs. If a system
includes disk groups with spinning disks, the disk groups must be removed before the all-flash array feature can be used.
If you are using SSDs and spinning disks and the first disk group is provisioned with spinning disks, then the system can be provisioned to
use spinning disks in virtual disk groups and use SSDs either in virtual disk groups or as read cache.
Internal disk management
SSDs use multiple algorithms to manage SSD endurance features. These include wear leveling, support for Unmap commands, and over-
provisioning to minimize write amplification.
Wear leveling
Wear leveling is a technique for prolonging the service life of some kinds of erasable computer storage media, such as the flash memory
used in SSDs. It attempts to ensure that all flash cells are written to or exercised as evenly as possible to avoid any hot spots where some
cells are used up faster than other locations. There are several different wear leveling mechanisms used in flash memory systems, each
with different levels of success.
Vendors have different algorithms to achieve optimum wear leveling. Wear leveling management occurs internal to the SSD. The SSD
automatically manages wear leveling, which does not require any user interaction.
Overprovisioning
The write amplification factor of an SSD is defined as the ratio of the amount of data actually written by the SSD to the amount of host or
user data requested to be written. This is used to account for the user data and activities like wear leveling. This affects wear leveling
calculations and is influenced by the characteristics of data written to and read from SSDs. Data that is written in sequential LBAs that are
aligned on 4KB boundaries results in the best write amplification factor. The worst write amplification factor typically occurs for randomly
written LBAs of transfer sizes that are less than 4KB and that originate on LBAs that are not on 4KB boundaries. Try to align your data on
4KB boundaries.
TRIM and UNMAP commands
A command (known as TRIM in the ATA command set and UNMAP in the SCSI command set) allows an operating system to inform an
SSD of the blocks of data that are no longer considered in use and can be wiped internally.
Data retention
Data retention is another major characteristic of SSDs that all SSD algorithms take into account while running. While powered up, the data
retention of SSD cells are monitored and rewritten if the cell levels decay to an unexpected level. Data retention when the drive is
powered off is affected by Program and Erase (PE) cycles and the temperature of the drive when stored.
Drive Writes per Day
DWD or DWPD refers to Drive Writes Per Day. Disk vendors rate SSD endurance by how many writes can occur over the lifetime of an
SSD. As lower-cost SSDs that support fewer drive writes per day become available, the cost benefit analysis of which SSDs to use is
highly dependent on your applications and I/O workload, as is the ratio of SSDs to conventional drives. In some environments, a ratio of
10% SSDs to 90% conventional drives, when combined with Dell EMC real-time tiering, can yield dramatic performance improvements.
Getting started
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