Transcript magnetic disk

Cache Memory
• Historically, CPUs have always been faster
than memories. In order to have a memory
as fast as a CPU it must be located on the
CPU chip. This makes the chip bigger and
more expensive.
 By combining a small fast memory with a large
slow memory, we can get the speed (almost) of
the fast memory for the price of the slow
memory. The fast memory is called a cache.
Cache Memory
 Basic idea: most heavily used memory words
are kept in the cache. When a memory word is
required, the CPU first looks in the cache. If the
word is not there, it looks to main memory.
• In order to be successful, a large percentage of the
words searched must be in the cache. We can ensure
this by exploiting the locality principle. When a
word is referenced, it and some of its neighbors are
brought into the cache, so next time it can be
accessed quickly.
Cache Memory
 Let c be the cache access time, m the main
memory access time, and h the hit ratio
(fraction of references satisfied out of the
cache). Then the mean access time is:
c + (1 - h) m
 As h approaches 1, all references can be
satisfied out of cache and the access time
approaches c. On the other hand, as h
approaches 0, the access time approaches c + m.
Cache Memory
 Using the locality principle, main memory and
cache are divided up into fixed-size blocks.
When referring to the cache, these blocks are
called cache lines. When a cache miss occurs,
the entire cache block is loaded.
 Instructions and data can either be kept in the
same cache (unified cache) or in separate
caches (split caches).
• There can also be multiple caches (on chip, off chip
but in the same package as the CPU, and farther
away).
Cache Memory
Memory Packaging and
Types
 From the early days of semiconductor
manufacture until the early 1990s, memory was
manufactured, bought, and installed as single
chips. At present, a different arrangement is
often used. A group of chips (typically 8 or 16)
is mounted on a printed circuit board and sold
as a unit. This unit is called a SIMM (Single
Inline Memory Module) or DIMM (Dual
Inline Memory Module), depending on
whether it has a row of connectors on one or
both sides of the board.
SIMM Modules
SIMM Modules
SIMM and DIMM Modules
 A typical SIMM configuration might have 8
chips with 4MB each on the SIMM for a total
of 32MB.
 The first SIMMs had 30 connectors and
delivered 8 bits at a time. Later SIMMs had 72
connectors and delivered 32 bits at a time. At
present, DIMMs are the standard way for
memory to be packaged. DIMMs can deliver 64
data bits at once. A physically smaller DIMM,
called an SO-DIMM (Small Outline DIMM)
is used in notebook computers.
SO-DIMM
Secondary Memory
• No matter how fast main memory is, it is
always too small.
• The traditional solution is a memory
hierarchy in which we have small amounts
of fast, expensive memory, and increasingly
larger amounts of slower, less expensive
memory. The storage capacity increases the
further we go down the hierarchy.
Memory Hierarchies
Magnetic Disks
 A magnetic disk consists of one or more
aluminum platters with a magnetizable coating.
They are typically 3 to 12 cm in diameter.
 A disk head floats over the surface of the disk.
The disk head can magnetize the surface or read
the bits previously stored.
 The circular sequence of bits written as the disk
makes a complete rotation is called a track.
Each track is divided into fixed-length sectors.
Magnetic Disks
 Sectors typically consist of a preamble (for
head synchronization), followed by 512 data
bytes. Following the data is an Error-Correcting
Code, either a Hamming code, or more
commonly, a code that can correct multiple
errors called a Reed-Solomon code. Between
consecutive sectors is a small intersector gap.
 Disk arms can move to different radial
distances from the spindle. At each distance, a
track is written.
Magnetic Disks
Magnetic Disks
 In order to achieve high quality, most disks are
sealed at the factory to prevent dust from
entering. These disks are called Winchester
disks.
 Most disks consist of multiple platters stacked
vertically. Each surface has its own arm and
head, but they move together. The set of tracks
at a given radial position is called a cylinder.
• Disk performance depends on seek time (moving
the arm to the right cylinder) and rotational latency
(spinning the disk to the right sector).
Magnetic Disks
Magnetic Disks
Magnetic Disks
 As disks rotate, they get hot and their physical
geometry changes. Thus, periodically they
undergo thermal recalibration (the heads are
forced all the way in or out).
• Special audio-visual disk drives do not have these
recalibrations since multimedia applications require
a constant bit stream.
 Each drive has an associated disk controller, a
chip that controls the drive. The disk controller
accepts commands from software, detects bad
sectors, buffers data, etc.
IDE Disks
 Early PCs had a disk controller on a plug-in
card. The OS read from and wrote to the disk
by putting parameters in CPU registers and then
calling the BIOS (Basic Input Output
System), located in the PCs ROM. The BIOS
then issued the machine instructions to load the
disk controller registers and initiate transfers.
 By the mid 1980s, the controller was no longer
on a separate card, but closely integrated with
the drive. These drives were called IDE
(Integrated Electronic Drives).
IDE Disks
IDE Disks
 BIOS calling conventions were not changed for
backward compatibility. Sectors were addressed
by giving head, cylinder, and sector numbers.
• 4 bits for the head, 6 bits for the sector, and 10 bits
for the cylinder give 1,032,192 possible sectors or a
maximum drive capacity of 504 MB (note that
sectors are numbered starting at 1, not 0).
• In order to get around these limits, the disk
controllers began to use different geometries that
couldn’t be addressed by the BIOS.
IDE Disks
 IDE drives evolved into EIDE drives
(Extended IDE) which support an addressing
scheme called LBA (Logical Block
Addressing) which numbers sectors from 0 to
228 - 1. The controller then converts LBA
addresses to head, sector, cylinder addresses,
but there is no 504 MB limit.
 EIDE controllers can also control four drives
instead of two, have a higher transfer rate, and
have the ability to control CD-ROM drives.
EIDE Drives
• Successors to EIDE were called ATA-3 and
ATAPI-4
 Still had a limit of 22829 bytes (128 GB)
 Subsequently, ATAPI-6 had a LBA size of 48
bits (max disk size of 128 PB).
 Speed increased from 16.67 MB/sec with EIDE
to 100 MB/sec with ATAPI-6
 ATAPI-7 uses serial ATA with a smaller drive
connector and lower power consumption
SCSI Disks
 SCSI disks are not different from IDE disks in
terms of how their cylinders, tracks, and sectors
are organized, but they have a different
interface and much higher transfer rates.
• SCSI is more than a hard disk interface, it is a bus to
which a SCSI controller and up to seven devices
(hard disks, CD ROMs, scanners, etc.) can be
attached.
 Each SCSI device has a unique ID, from 0 to 7
(15 for wide SCSI). Each device has two
connectors, one for input and one for output.
SCSI Disks
 Cables connect the output of one device to the
input of the next, in series. The last device in
the series must be terminated to prevent
reflections from interfering with other data.
Typically, the controller is on a plug-in card.
 SCSI controllers and devices can operate either
as initiators or as targets. Usually, the controller
issues commands to the disks and peripherals.
• Multiple SCSI devices can run at once using bus
arbitration. IDE and EIDE allow only one active
device at a time.
SCSI Disks
RAID
 Parallel processing is increasingly used to speed
up CPU performance. Likewise, parallel I/O
can be used to speed up I/O.
 RAID (Redundant Array of Inexpensive
Disks) is a set of six specific disk organizations
that can be used to improve disk performance,
or reliability, or both.
• Most RAIDs consist of a RAID SCSI controller plus
a box of SCSI disks that appear to the OS as a single
large disk. No software changes are required to use
the RAID.
RAID
 The data in a RAID are distributed over the
drives, to allow parallel operation. Several
schemes were devised by Patterson and are
known as RAID level 0 through RAID level 5.
 RAID level 0 stripes data over multiple drives
in round robin fashion. In this way, a block of
data can be read from multiple drives, working
in parallel. This works best when data is
requested in large chunks. The controller must
coordinate the request and issue the individual
requests to the disks.
RAID
• RAID level 0 is less robust than using a single disk,
since the failure of any disk leads to failure for the
RAID.
• RAID level 1 is a true RAID. It duplicates all the
disks so that there are four primary and four backup
disks. On a write, a strip is written twice. On a read,
either copy can be read. Fault tolerance is excellent.
• RAID level 2 works on a word basis, possibly even
a byte basis. All drives must be synchronized and
there must be a large number of drives.
RAID
 The Thinking Machine CM-2 used this scheme,
taking a 32-bit word and adding 6 parity bits to
form a 38-bit Hamming word, plus an extra bit
for word parity. Each word was spread over 39
disks.
 RAID level 3 is a simplified version of level 2.
A single parity bit is computed for each data
word and written to a parity drive. As in level 2,
the drives must be exactly synchronized.
Thinking Machines CM-2
RAID
 RAID levels 4 and 5 work with strips again,
with parity. RAID level 4 writes a parity strip
(computed from the strips on each of the other
disks) on an extra drive. If a drive crashes, the
lost bytes can be recomputed from the parity
drive.
 RAID level 5 distributes the parity bits
uniformly over all the disks so that the parity
drive does not become a bottleneck. This makes
recovery more complex, however.
RAID
RAID
CD-ROM
 Optical disks are attractive storage devices
since they have much higher recording densities
than magnetic disks.
 Audio CDs code data by burning pits into the
surface of the CD. The unburned areas between
the pits are called lands. The data is recorded
on a spiral groove. The CD is then covered with
a protective material. The data on the CD can
then be read with a laser. A pit/land or land/pit
transition records a 1, and its absence is a 0.
CD-ROM
CD-ROM
 In 1984, the CD-ROM standard for storing
computer data on CDs was published. CDROMs were mechanically and optically
compatible with audio CDs.
 Error correction/detection coding was added
using 14-bit symbols to encode one data byte.
 42 consecutive symbols form a 588-bit frame.
Each frame contains 192 data bits. The
remaining 396 bits are for error correction and
control.
CD-ROM
 98 frames are combined into a CD-ROM sector
with a 16-byte preamble and a 288-byte errorcorrecting code.
 Single-bit errors are corrected at the lowest
level, short burst errors at the frame level, and
any residual errors at the sector level.
 CD-ROMs are much slower than highperformance magnetic disks.
• The CD-ROM file system is called High Sierra (IS
9660) and can be read on many different computers.
CD-ROM
CD-R Disks
• CD-R (CD-Recordable) disks allow for data to be
written (once) to a CD. These disks use gold rather
than aluminum for the reflective layer and rather
than pits and lands, use a layer of dye which can be
affected by the CD-R laser to simulate the pits and
lands.
• Multisession CDs allow for incremental writing
(don’t write the contents at the start of the disk).
• CD-RW (CD-ReWritable) uses an alloy for the
recording layer rather than dye. CD-RW blanks are
more expensive than CD-R blanks.
CD-R Disks
DVD
 CD-ROM disks do not have a high enough
capacity to store digital videos, so a new
standard reflecting technology advances of the
last 20 years called DVD (Digital Versatile
Disk) has been introduced. The basic design is
the same as CDs, but with:
• smaller pits
• a tighter spiral
• a red laser
 This increases the capacity to 4.7 GB, enough
to hold 133 minutes of MPEG-2 video.
DVD
 The red laser means that in order to read both
DVD and CD disks, drives must have a second
laser.
 In order to increase capacity, double-sided and
dual-layer formats have been defined.
• The dual-layering has a reflective layer at the
bottom, topped with a semireflective layer.
Depending on where the laser is focused, it bounces
off one layer or the other.
• Double-sided disks are made by gluing two disks
together, back-to-back.
DVD
Blu-Ray
• The next generation optical disk format is
called Blu-Ray
 Uses blue, rather than red, laser
 Smaller pits and lands
 Single sided storage of 25 GB, double sided at
50 GB
 4.5 MB/sec data rate
 To be used for recording HDTV