3Magnetic DiskMagnetic disks are the foundation of external memory on virtually all computer systems.A disk is a circular platter constructed of nonmagnetic material, called the substrate, coated with a magnetizable material.The substrate has been an aluminum material.Glass substrates have been introduced.
4Read and Write Mechanisms Recording & retrieval via conductive coil called a headMay be single read/write head or separate onesDuring read/write, head is stationary, platter rotatesWrite- Current through coil produces magnetic field- Pulses sent to head- Magnetic pattern recorded on surface below
5Read and Write Mechanisms Read (traditional)- Magnetic field moving relative to coil produces current- Coil is the same for read and writeRead (contemporary)- Separate read head, close to write head- Partially shielded magneto resistive (MR) sensor- Electrical resistance depends on direction of magnetic field- High frequency operation- Higher storage density and speed
7Data Organization and Formatting Concentric set of rings, called tracks each track has the same width as the head. There are thousands of tracks per surface.- Gaps between tracks- Reduce gap to increase capacity- Same number of bits per track (variable packing density)- Constant angular velocityTracks divided into sectorsMinimum block size is one sectorMay have more than one sector per block (track)
9Disk VelocityBit near centre of rotating disk passes fixed point slower than bit on outside of diskIncrease spacing between bits in different tracksRotate disk at constant angular velocity (CAV)- Gives pie shaped sectors and concentric tracks- Individual tracks and sectors addressable- Move head to given track and wait for given sector- Waste of space on outer tracks- Lower data densityCan use zones to increase capacity- Each zone has fixed bits per track-More complex circuitry
11Finding Sectors Must be able to identify start of track and sector Format disk- Additional information not available to user- Marks tracks and sectorsAn example of disk formatting is shown in Figure 6.4. In this case, each track contains 30 fixed-length sectors of 600 bytes each.Each sector holds 512 bytes of data plus control information useful to the disk controller.
12The ID field is a unique identifier or address used to locate a particular sector. The SYNCH byte is a special bit pattern that delimits the beginning of the field. The track number identifies a track on a surface.The head number identifies a head, because this disk has multiple surfaces.The ID and data fields each contain an error detectingcode.
15Physical Characteristics Head Motion- Fixed head (one read write head per track and Heads mounted on fixed ridged arm)- Movable head(one per surface and mounted on a movable arm).Disk Portability- Nonremovable disk (fixed )- Removable disk (Can be removed from drive and replaced withanother disk, provides unlimited storage capacity, and easy data transfer between systems)
16Physical Characteristics Sides- Single sided- double (usually) sidedPlatters- Single platters- multiple platter (One head per side, heads are joined and aligned, aligned tracks on each platter form cylinders and data is striped by cylinder:1. Reduces head movement2. Increases speed (transfer rate)
19Physical Characteristics Head mechanism- Contact (Floppy disk) 8”, 5.25”, 3.5”Small capacity up to 1.44Mbyte (2.88M never popular)Slow, universal, cheap- Fixed gap- Flying (Winchester)Developed by IBM in Winchester (USA), Sealed unitOne or more platters (disks), Very small head to disk gap universal, cheap, Fastest external storageGetting larger all the time. 250 Gigabyte now easily available
20RAID Redundant Array of Independent Disks (RAID Redundant Array of Inexpensive Disks7 levels in common useNot a hierarchySet of physical disks viewed as single logical drive by O/SData distributed across physical drivesCan use redundant capacity to storeparity information
21RAID These levels share three common characteristics: 1. RAID is a set of physical disk drives viewed by the operating system as a single logical drive.2. Data are distributed across the physical drives of an array in a scheme known as striping, described subsequently.3. Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure.The details of the second and third characteristics differ for the different RAID levels. RAID 0 and RAID 1 do not support the third characteristic.
22RAID 0 No redundancy Data striped across all disks Round Robin stripingIncrease speed- Multiple data requests probably not on same disk- Disks seek in parallel- A set of data is likely to be striped across multiple disks
26RAID 2 RAID levels 2 and 3 make use of a parallel access technique. In a parallel access array, all member disks participate in the execution of every I/O requestthe individual drives are synchronized so that each disk head is in the same position on each disk at any given time.RAID 2 requires fewer disks than RAID 1
27RAID 3 RAID 3 requires only a single redundant disk. Employs parallel access, with data distributed in small stripsCan achieve very high data transfer rates.Only one I/O request can be executed at a time
28RAID 4 Each disk operates independently Good for high I/O request rate Large stripesBit by bit parity calculated across stripes on each diskParity stored on parity disk
30RAID 5 Like RAID 4 Parity striped across all disks Round robin allocation for parity stripeAvoids RAID 4 bottleneck at parity diskCommonly used in network servers
31RAID 6 Two parity calculations Stored in separate blocks on different disksUser requirement of N disks needs N+2High data availability- Three disks need to fail for data loss- Significant write penalty, because each write affects two parity blocks.