Storage subsystem jargons

Posted: February 4th, 2013 | Author: | Filed under: FreeBSD, storage | Tags: , , , , , | No Comments »

Standards in order of their creation/availability:
SCSI (Serial Computer System Interface), oldest
ATA (IDE) : AT Attachment, later became PATA (Parallel ATA)
SATA (Serial ATA) : better than SCSI
SAS (Serial Attached SCSI) : better than SATA

SCSI/SATA controls RAID.

SAS : costlier, better for critical functions, server applications
SATA: cheaper, used for personal computers

SAS controller can access SATA drives but that’s not true the other way around.

SAS and SATA drives can operate in the same environment while SCSI and ATA cannot. For example, using faster SAS drives for primary storage and offloading older data to cheaper SATA disks in the same subsystem, something that could not be achieved with SCSI and ATA.

CAM (Common Access Method) : a specification for SCSI
CAM provides a formal description of the interfaces in a SCSI subsystem.
Benefits: providing round-robin prioritized transaction queuing, guaranteed transaction ordering even during error recovery, and a straight forward error recovery model that increases system robustness.

CAM is not perfect, a lot of issues related to implementation of the standard.

More to come about CAM and RAID.


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