Information Technology Reference
In-Depth Information
to time it takes the disk to move the head from one track to another or to switch
from the head on one surface to the head on another one. This staggering is
called track skewing .
.Definition: track skewing
To increase storage density and disk capacity, disk manufacturers make
tracks and sectors as thin and small as possible. If there are imperfections
in a sector, then that sector may be unable to reliably store data. Manufac-
turers therefore include spare sectors distributed across each surface. The disk
rmware or the le system's low-level formatting can then use sector sparing to
Denition: sector sparing
remap sectors to use spare sectors instead of faulty sectors. Slip sparing helps
Denition: slip sparing
retain good sequential access performance by remapping all sectors from the
bad sector to the next spare, advancing eash logical sector in that range by one
physical sector on disk.
Disk drives often include a few MB of buffer memory , memory that the
Denition: buffer memory
disk's controller uses to buer data being read from or written to the dsk, for
track buffering, and for write acceleration.
Track buffering improves performance by storing sectors that have been read
Denition: track buffer
by the disk head but not yet requested by the operating system. In particular,
when a disk head moves to a track, it may have to wait for the sector it needs
to access to rotate under the disk head. While the disk is waiting, it reads
unrequested sectors to its rack buffer so that if the operating system requests
thoses sectors later, they can be returned immediately.
Write acceleration stores data to be written to disk in the disk's buer
Denition: write
acceleration
memory and acknowleges the writes to the operating system before the data is
actually written to the platter; the disk firmware flushes the writes from the
track buffer to the platter at some later time. This technique can significantly
increase the apparent speed of the disk, but it carries risks|if power is lost
before buffered data is safely stored, then data might be lost.
Server drives implementing the SCSI or Fibre Channel interfaces and in-
creasing numbers of commodity drives with the Serial ATA (SATA) interface
implement a safer form of write acceleration with tag command queuing (TCQ)
Denition: tag command
queuing
(also called native command queuing (NCQ) for SATA drives.) TCQ allows an
Definition: native
command queuing
operating system to issue multiple concurrent requests to the disk and for the
disk to process those requests out of order to optimize scheduling, and it can be
configured to only acnowledge write requests when the blocks are safely on the
platter.
12.1.1
Disk access and performance
Operating systems send commands to a disk to read or write one or more consec-
utive sectors. A disk's sectors are identied with logical block addresses (LBAs)
Denition: logical block
addresses
that specify the surface, track, and sector to be accessed.
To service a request for a sequence of blocks starting at some sector, the disk
