Definitions of common Xsan terms
Affinity
Array
Bandwidth
Block
- A lump of something of a particular size. e.g. "Data is stored on my disk in blocks of 512K."
Bonjour
- An often-misunderstood collection of services to assist in network configuration, computer-discovery, and service-discovery. "Misunderstood" refers to both users who blame all sorts of things on Bonjour, and to engineers who think it works in a way it doesn't. For some history and a good overview, see http://www.zeroconf.org/ or Wikipedia.
- A part of Bonjour is self-assigned IPs in the range 169.254.0.0 - 169.254.255.254. An address in this range is automatically and randomly chosen for a computer's (Mac and Windows) primary interface when an IP address isn't otherwise available through DHCP or manual configuration.
- Bonjour also includes multicast DNS, which allows a group of networked computers (usually Macs) to create their own distributed DNS system. You can't really turn this off.
- Bonjour also includes DNS-SD, which can advertise services through multicast DNS.
Cache
- A repository of data, usually stored in a quickly accessible location, in order to provide quick access. Caches on hardware components are usually in the form of volatile RAM.
- Caches in operating systems, such as Mac OS X, are in the form of flat files that store information that the system often accesses. Caching in Mac OS X is one of the reasons that the operating system is nimble and speedy. Caching can also cause issues when the information contained within the cache is old, incorrect or corrupt.
- Hardware caches allow data I/O operations to optimize because they allow I/O confirmation statements to be sent, even though the data has not reached its final destination. For example, write caches built into hard drives can be enabled, allowing the hard drive to respond that it has written incoming data onto its platters, even though the data still resides in cache. A serious issue can occur, therefore, if the confirmation statements gets sent and power is cut before the cache can flush its data to the correct location.
Client (Xsan client)
Controller (metadata)
Controller (Xserve Raid component)
Cvadmin
- A command-line tool for starting and stopping Xsan volumes. The "status" command is a useful way of seeing the active and standby metadata controllers for each volume. The command lives at /Library/Filesystems/Xsan/bin/.
Ethernet
Ethernet Switch
Fabric
Failover
Fibre Channel
Fibre Channel Switch
HBA
- "Host Bus Adapter." The industry name for a expansion card that provides an additional connectivity protocol to a computer motherboard. Cards that provides Fibre Channel, SATA or even additional Firewire ports are all different kinds of HBAs.
Hot Spare
- A disk in a RAID array that waits unused until a disk failure. The RAID controller then promotes that disk to the RAID, using parity data from the other array drives to create a clone of the failed disk.
- Since an array with a hot spare will rebuild itself to a fault-tolerant state as quickly as possible, the operator can respond less urgently to failed disk alarms. However, the bad drive that has been demoted should be replaced with a new hot spare as soon as possible.
- In Xserve RAIDs, blank disks installed but unassigned to an array are automatically tagged as hot spares. Disks that were part of a foreign array may need to be manually marked as spares. In other RAIDs, disks may need to be manually marked as spares.
IPV6
Latency
LUN
- "Logical Unit Number." A grouping of individual drive mechanisms into a single storage entity, or unit. Usually, a RAID configuration is what governs this Unit, and when presented to a computer, the OS assigns it a device number. Either a software program within an OS, or an external hardware device, such as an HBA or a controller within a box with a series of drive mechanism, creates the RAID array.
RAID
RSCN
Switch
Xsan Admin
Xsan Tuner
- An application, written by the Xsan development team at Apple on or around September 7, 2005, that provided a series of I/O tests in order to test total bandwidth availability of the SAN. Users could select testing for a particular storage pool, emulate the pull for certain kinds of video codec streams through Final Cut Pro's buffering system, or do straight wire-speed unix I/O commands.
- Two issues now plague Xsan Tuner: Firstly, it is still written in PPC code, and so it runs in Rosetta on any Intel Mac. Because of this, its results are inaccurate and its Storage Pool testing option is broken. Secondly, since it writes its test data always to the root of the Xsan Volume, the first user that runs it creates a folder that is usually read-only to other users. Therefore, until the permissions of that folder are set to read/write for a collaborative group or the world, multi-system tests cannot begin.
Xserve
Xserve Raid
Xserve Raid Admin