Chapter 20.
Solid-State Disk Deployment Guidelines
Solid-state disks (SSD) are storage devices that use NAND flash chips to persistently store data. This sets them apart from previous generations of disks, which store data in rotating, magnetic platters. In an SSD, the access time for data across the full Logical Block Address (LBA) range is constant; whereas with older disks that use rotating media, access patterns that span large address ranges incur seek costs. As such, SSD devices have better latency and throughput.
Not all SSDs show the same performance profiles, however. In fact, many of the first generation devices show little or no advantage over spinning media. Thus, it is important to define classes of solid state storage to frame further discussion in this section.
SSDs can be divided into three classes, based on throughput:
The first class of SSDs use a PCI-Express connection, which offers the fastest I/O throughput compared to other classes. This class also has a very low latency for random access.
The second class uses the traditional SATA connection, and features fast random access for read and write operations (though not as fast as SSDs that use PCI-Express connection).
The third class also uses SATA, but the performance of SSDs in this class do not differ substantially from devices that use 7200rpm rotational disks.
For all three classes, performance degrades as the number of used blocks approaches the disk capacity. The degree of performance impact varies greatly by vendor. However, all devices experience some degradation.
To address the degradation issue, the ATA specification outlines a new command: TRIM. This command allows the file system to communicate to the underlying storage device that a given range of blocks is no longer in use. The SSD can use this information to free up space internally, using the freed blocks for wear-leveling.
Enabling TRIM support is most useful when there is available free space on the file system, but the file system has already written to most logical blocks on the underlying storage device. For more information about TRIM, refer to its Data Set Management T13 Specifications from the following link:
Not all solid-state devices in the market support TRIM.
20.1.
Deployment Considerations
Because of the internal layout and operation of SSDs, it is best to partition devices on an internal erase block boundary. Partitioning utilities in Red Hat Enterprise Linux 6 chooses sane defaults if the SSD exports topology information. This is especially true if the exported topology information includes alignment offsets and optimal I/O sizes.
However, if the device does not export topology information, Red Hat recommends that the first partition be created at a 1MB boundary.
In addition, keep in mind that logical volumes, device-mapper targets, and md targets do not support TRIM. As such, the default Red Hat Enterprise Linux 6 installation will not allow the use of the TRIM command, since this install uses DM-linear targets.
Red Hat also warns that software RAID levels 1, 4, 5, and 6 are not recommended for use on SSDs. During the initialization stage of these RAID levels, some RAID management utilities (such as mdadm) write to all of the blocks on the storage device to ensure that checksums operate properly. This will cause the performance of the SSD to degrade quickly.
At present, ext4 is the only fully-supported file system that supports TRIM. To enable TRIM commands on a device, use the mount option discard. For example, to mount /dev/sda2 to /mnt with TRIM enabled, run:
mount -t ext4 -o discard /dev/sda2 /mnt
By default, ext4 does not issue the TRIM command. This is mostly to avoid problems on devices which may not properly implement the TRIM command. The Linux swap code will issue TRIM commands to TRIM-enabled devices, and there is no option to control this behaviour.
