Computer data storage - Some glossary

Computer data storage Failure, error, and how to deal (concepts) Noticing errors and failure Reading SMART reports Partitioning and filesystems ZFS notes Network storage RAID notes mdadm notes, aacraid notes, OMSA notes, LSI notes General & RAID performance tweaking SSD notes LVM notes Some glossary Semi-sorted

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

DAS (Direct Attached Storage) - storage not physically in a host, but still connected directly to just it

often means a connection like SAS, SCSI, Fibre Channel, eSATA or such (various are easily be faster than networking)

SAN (Storage Area Network)

storage attached via a (local area) network, usually in a lower-level way

often exposes a block device(verify)

often there to centralized a large amount of storage for a network of computers.

you arguably do SAN for consolidation and flexibility first, and performance may be secondary

e.g. iSCSI is a SAN protocol

NAS (Network Attached Storage)

storage attached via a (local area) network, usually in a higher-level way

Can refer to simple file sharing (SMB, NFS and such), to specific applicances.

often exposes a filesystem

more of an appliance (SAN being more of a service)

• S.M.A.R.T.: Self-Monitoring Analysis and Report Technology, meant to monitor drive health and warn about imminent failure.

(Compliance with this standard possibly seems to vary per manufacturer, and also probably somewhat per drive design)(verify)

see also Computer data storage - Reading SMART reports

• Command queueing refers to planning an order of read and/or write commands order so that the physical properties of the storage is taken into account (e.g. for platter this might be head position and rotational position)

Command queueing standards means that the storage can decide how to serve commands; without that you can make the OS estimate what reordering might help, but it can only guess about the physical properties of the storage so probably not do quite as well as if it is natively supported.

In the best case a series of operations finishes faster, in the worst case it makes no difference and the overhead (fairly little for NCQ, while for TCQ it depends somewhat on the implementation/technology) makes things a little slower. The practical value of command queueing depends largely on the average and worst case influence.

TCQ: Tagged Command Queueing: Done by the driver (verify)

NCQ: Native Command Queueing: Done by the drive (SCSI/SATA feature, optional?(verify)).