Types of Disk

When you buy a server or storage array these days, you often have the choice between three different kinds of hard drives:

• Serial Attached SCSI (SAS)
• Near Line SAS (NL-SAS)
• Serial ATA (SATA).

SAS

Also known as Tier-1, these 10K & 15K RPM SAS drives provide the online enterprise performance and availability needed for mission critical application

• General Standard in storage these days
• Most reliable
• Generally high performing
• Lower BER (Bit Error Rate) than other types of disk.1 in 10^16 bits
• SAS disks have a mean time between failure of 1.6 million hours compared to 1.2 million hours for SATA
• SAS disks/controller pairs also have a multitude of additional commands that control the disks and that make SAS a more efficient choice than SATA.

Nearline SAS?

An NL-SAS disk is a bunch of spinning SATA platters with the native command set of SAS. While these disks will never perform as well as SAS thanks to their lower rotational rate, they do provide all of the enterprise features that come with SAS, including enterprise command queuing, concurrent data channels, and multiple host support.

• NL-SAS drives are enterprise SATA drives with a SAS interface, head, media, and rotational speed of traditional enterprise-class SATA drives with the fully capable SAS interface typical for classic SAS drives.”
• Same speed really as SATA. While these disks will never perform as well as SAS thanks to their lower rotational rate, they do provide all of the enterprise features that come with SAS
• Good if you need 1TB drives in a SAS server, say for backups.
• Not good for first or primary storage in a SAS based server.
• Enterprise/tagged command queuing. Simultaneously coordinates multiple sets of storage instructions by reordering them at the storage controller level so that they’re delivered to the disk in an efficient way.
• Concurrent data channels. SAS includes multiple full-duplex data channels, which provides for faster throughout of data.
• Multiple host support. A single SAS disk can be controlled by multiple hosts without need of an expander.
• The BER is generally 1 in 10^15 bits.
• NL-SAS disks rotate at speeds of 7200 RPM… the same as most SATA disks, although there are some SATA drives that operate at 10K RPM.

SATA

Also known as Tier-2: these Nearline, Business Critical 5.4K and 7.2K RPM drives combine specific design and manufacturing processes for hard drives rated at 24x7x365 operations for true enterprise duty cycles. The main emphasis is on an exceptional dollars/GigaByte advantage over Tier 1 storage

• It doesn’t perform as well as SAS and doesn’t have some of the enterprise benefits of NL-SAS
• Used for large cheap capacity over performance