Group 6 Presentation Andres Reyes Jesus Amundarain Alberto Lacaci Shawn Gunness Andres Marcial Stephen Creaville Jimmy Jean-Paul

Magnetic Disk

The primary computer storage device It consists of magnetically coated disks Glass is the used as the substrate. Advantages: o Increases reliability o Reduces surface defects o More resistance to damage o Better stiffness

Read and Write Mechanisms For writing, the surface of the disk or tape is moved past the read/write head. By discharging electrical impulses at the appropriate times, bits are recorded as tiny, magnetized spots of positive or negative polarity. For reading, the surface is moved past the read/write head, and the bits that are present induce an electrical current across the gap.

Read (contemporary) Most modern disk drives use magneto resistive sensor for reading, but use inductive coils for writing. High frequency operation o Higher storage and speed

Data Organization and Formatting Tracks and Spots The disk surface is divided into concentric tracks (circles within circles). The thinner the tracks, the more storage. The data bits are recorded as tiny magnetic spots on the tracks. The smaller the spot, the more bits per inch and the greater the storage. Sectors Tracks are further divided into sectors, which hold a block of data that is read or written at one time; for example, READ SECTOR 782, WRITE SECTOR In order to update the disk, one or more sectors are read into the computer, changed and written back to disk.

Track and Sectors

Disk Velocity The bit located near centre of rotating disk passes fixed point slower than the bit on the outside of the disk Constant angular velocity (CAV) keeps the disk spinning at a fixed rate. This means the heads cover more distance per unit of time on the outer tracks than on the inner tracks. o Individual tracks and sectors addressable o Lower data density Can use zones to increase capacity o Each zone has fixed bits per track

Finding Sectors Find start of track and sector Format disk o Marks tracks and sectors

Winchester Disk Format An early removable disk drive from IBM that put the heads and platters (disks) in a sealed unit for greater speed. The drive had one permanent and one removable spindle, each holding 30MB.

Characteristics Fixed head o One read write head per track o Heads mounted on fixed ridged arm Movable head o One read write head per side o Mounted on a movable arm Removable disk o Can be removed from drive and replaced with another disk o Provides unlimited storage capacity o Easy data transfer between systems Nonremovable disk o Permanently mounted in the drive

Multiple Platter One head per side Heads are joined and aligned Aligned tracks on each platter form cylinders Data is striped by cylinder o reduces head movement o Increases speed (transfer rate)

Floppy Disk A floppy disk is a data storage medium that is composed of a disk of thin, flexible ("floppy") magnetic storage medium encased in a square or rectangular plastic shell. Floppy disks in 8 inch, 5¼ inch and 3½ inch. Small capacity o Up to 1.44Mbyte (2.88M never popular)

Speed Seek time o Moving head to correct track (Rotational) latency o Waiting for data to rotate under head Access time = Seek + Latency Transfer rate

Raid system Drive arrays are ways to share resources The most common drive arrays are RAID RAID stands for Redundant Arrays of Inexpensive Disks

RAID 0 Stripped Disk Array without Fault Tolerance (Nonredundant) Does not include redundancy to improve performance Increase probability that requests of data being issued of different blocks could be issued on parallel, reducing the I/O queuing time. RAID level 0 can be implemented for two situations: High Data Transfer Capacity and High I/O Request Rate

RAID 1 Mirroring and Duplexing (Mirrored) Redundancy is achieved by duplicating all the data A read request can be serviced by either of the two disks. The controller reads alternate sectors from each drive. Time is effectively reduced the data read time. Write time is dictated by the slower of the two writes Recovery from failure is achieved accessing the second drive Main disadvantage is cost.

RAID 2 Data Striping with Error Recovery Uses multiple disks for error- detection and correction functions The number of redundant disks is proportional to the log of the number of data disks. A complex error-detection and correction algorithm is employed. Big hardware overhead, thus is not normally used with microcomputer systems

RAID 3 Parallel Transfer with Parity Striping (Bit-interleaved parity) Same organization as RAID 2 but only a single redundant disk is required, independently of the disk array size A redundancy is added just in case of a disk failure so data lost can be reconstructed using the parity disk Because data are striped in very small strips, RAID 3 can achieve very high data transfer rates Only one I/O request can be executed at a time. Thus, in a transaction-oriented environment, performance suffers If an error occurs, the controller reads the array again to verify the error. This is a time-consuming low-efficiency method of error correction

RAID 4 Independent Data Disks with Shared Parity Disk (Block-level parity) Controller interleaves sectors across the drives in the array Independent access technique for high I/O request rates I/O requests satisfied in parallel Only one parity-checking drive is allotted for error control

Bitwise operation XOR properties: A xor A = 0 1 xor 1 = 0 1 xor 0 = 1 0 xor 0 = 0 Example: 1110 xor 0111 = 1001 In general, A xor B xor C xor D xor E is TRUE if an ODD number of variables are TRUE XOR is used in RAID 3-6 for creating parity information. It is possible to bake up information like two bytes, and coming from different hard drives if we xor them and store the result in another hard drive

Data Recovery Suppose the following hard drives in a RAID 3-6 scheme: X3(i) = parity drive; X2(i) = , and X1(i) = , data drives, xored together X3(i) = X2(i) xor X1(i) = xor In the event of a failure, for example X1(i) fails, then is possible to recover the information adding X1(i) xor X3(i) to the equation as follows X3(i) xor X1(i) xor X3(i) = X2(i) xor X1(i) xor X1(i) xor X3(i) Because A xor A =0 we can eliminate X3(i) in the left member and X1(i) in the right member, leaving the equation as: X1(i) = X2(i) xor X3(i) Substituting values X1(i) = xor X1(i) = , so the data has been recovered.

RAID 5 Independent Data Disks with Distributed Parity Blocks Similar organization as RAID 4 Alters the RAID 4 specification by allowing the parity function to rotate through the different drives Error checking an correction are the function of all the drives The distribution of parity strips across all drives avoids the potential I/O bottleneck found in RAID 4 Very popular because it can be used on small arrays, and it has a high level of error recovery built in.

RAID 6 Independent Data Disks with Two Independent Distributed Parity (Dual Redundancy) Parity is calculated and stored in separate blocks and different disks Requires at least two additional drives to operate Example: if a data user needs N disks, RAID 6 array will be N+2 disks Relatively expensive, but it provides an extremely high fault tolerance level for critical applications

Raid comparison

Raid comparison (continued)

Optical Memory CD-ROM CD-Recordable (CD-R) CD-R/W DVD

Optical Storage CD-ROM Originally used audio recording 650Mbytes giving over 70 minutes audio Polycarbonate coated with highly reflective coat, usually aluminium Data stored as pits Read by reflecting laser Constant packing density Constant linear velocity

CD Operation

CD-ROM Drive Speeds Audio is single speed o Constant linier velocity Other speeds are quoted as multiples e.g. 48x Quoted figure is maximum drive can achieve

CD-ROM Format Mode 0=blank data field Mode 1=2048 byte data+error correction Mode 2=2336 byte data

With the use of Constant Linear Velocity Random Access on CD-ROM  Difficult  Does not go to exact position desired  Read Random address is difficult

CD-ROM FOR: o Relatively Large capacity o Easy to mass produce o Removable o Robust/Durable AGAINST: o Slow o Read only (cannot be updated) o Access time is longer than Mag. Disk Drive

Other Optical Storage Advantages CD-Recordable (CD-R) o WORM (WRITE ONCE READ MULTIPLE) o Now affordable o Compatible with CD-ROM drives CD-RW o Erasable o Getting cheaper o Mostly CD-ROM drive compatible o Phase change  Material has two different reflectivities in different phase states

DVD Digital Video Disk o Used to indicate a player for movies  Only plays video disks (however could be used for any storage application) Digital Versatile Disk o Used to indicate a computer drive  Will read computer disks and play video disks

DVD - technology Multi-layer Very high capacity (4.7G per layer) Full length movie on single disk o Using MPEG compression Movies carry regional coding

Magnetic Tape

The first kind of secondary memory was invented for recording sound by Fritz Pfleumer in 1928 in Germany. Magnetic recording is used for audio, video, and computer data storage. Tape systems use the same reading and recording techniques as disk systems. Data on the tape are structured as a number of parallel tracks running length wise. Early tape systems used nine tracks, which made it possible to store data one byte at a time, with an additional parity bit as the ninth track. Later tape systems used 18 or 36 tracks. Parallel recording is recording data in digital word or double word. Serial recording is data laid out as sequence of bits along each track. The recording technique used in serial tapes is referred to as serpentine recording. In this technique, when data are being recorded, the first set of bits is recorded along the whole length of the tape. When the end of the tape is reached, the heads are repositioned to record a new track, and the tape is again recorded on its whole length, this time in the opposite direction. Magnetic tape have been used in different formats such as open reels, UNIVAC, DEC, Cartridges, IBM formats and cassettes. Linear Tape-Open (LTO) Tape Drives o Developed late 1990s o Open source alternative to proprietary tape systems As of 2008, the highest capacity tape cartridges can store 1 TB of uncompressed data. (Sun Storage Tek T1000B & IBM TS 1130)

Advantages & Disadvantages Main advantages Relatively cheap Can hold in excess of 1 TB of data (it is possible to get devices that will back up over 5 TB of data) Backup capacity is easily expanded by simply using more tape. Main disadvantages Serial access - this means that in order to get to something on the tape, you have to go through everything that comes before it. Think of it like a video tape. The program that you want to watch might have been recorded half way through the tape, but you have to wind forward right from the start to get to it. Slow - Because of serial access, it is relatively slow to find the data that you need (however in an exam question, don't just say 'slow', compare it to a storage device with faster access. Specialist hardware is needed to read the tapes. Most standard PCs do not come with the hardware required to use them. Not suitable for heavy use - As you may realize from video tapes at home - they are a bit fragile with a tendency to stretch and tangle! Tape is best suited for back- up purposes.

Linear Tape-Open (LTO) Tape Drives

Review Questions Magnetic Disk: - What are the advantages of using glass as a substrate? -What is the differences between latency and seek time? RAID: - What is RAID and how many types are there? -What is the importance of parity bit? Optical Memory: -Name four types of optical memory? -With the use of Constant Linear Velocity is it easy or difficult to access random data on CD-ROM? Magnetic Tape: - What are the main advantages of magnetic tape? -What are magnetic recording used for?