1 COMP 206: Computer Architecture and Implementation Montek Singh Wed., Nov. 20, 2002 Topic: Storage Systems (Disk Technology)

2 Disk Systems: Characteristics  Capacity [bytes] Mainframes typically have  3.7 GB of disk storage per MIPS Mainframes typically have  3.7 GB of disk storage per MIPS PCs rend to require  5 MB of disk storage per MIPS PCs rend to require  5 MB of disk storage per MIPS  Bandwidth (throughput) Bytes transferred per unit time Bytes transferred per unit time  Service rate Number of service requests satisfied per unit time Number of service requests satisfied per unit time Supercomputer I/O requests tend to involve large amounts of data, transaction processing systems tend to involve small ones Supercomputer I/O requests tend to involve large amounts of data, transaction processing systems tend to involve small ones  Response time (latency) Time between start and completion of an event Time between start and completion of an event  Cost [$/MB]

3 Parameters of A Single Disk Drive

4 Technical Details (1)  Platters 2-4mm thick, made of an aluminum alloy 2-4mm thick, made of an aluminum alloy Typically 1-5 platters in a hard disk Typically 1-5 platters in a hard disk Both sides of each platter generally used Both sides of each platter generally used One side of one platter dedicated to information to guide the servomechanisms that control speed of rotation and head movement One side of one platter dedicated to information to guide the servomechanisms that control speed of rotation and head movement Each recording surface has its own read-write head Each recording surface has its own read-write head  Only one operational at any time  Data transfer to and from disk is bit-serial  Rotation Speed of rotation carefully controlled by servomechanism Speed of rotation carefully controlled by servomechanism  E.g., 3600 rpm  1% or even  1 rpm All disk drives are synchronized in some disk arrays All disk drives are synchronized in some disk arrays  Angular positions are identical (within tolerance) at any time

5 Technical Details (2)  Flying height Read/write heads do not touch rotating disk surface Read/write heads do not touch rotating disk surface Careful aerodynamic design keeps small constant distance (  0.2  ) Careful aerodynamic design keeps small constant distance (  0.2  ) Disk moves with approximate linear speed of 700 inches/sec Disk moves with approximate linear speed of 700 inches/sec  18m/s, 40 miles/hour Smaller flying height  higher writing density Smaller flying height  higher writing density  Platters and heads sealed hermetically in clean space for this reason  Called Winchester drives for historical reasons  “Parking the heads” Before disk is allowed to slow down and stop, heads are moved over an area of the disk not used for recording, where they are allowed to come into contact with the disk surface Before disk is allowed to slow down and stop, heads are moved over an area of the disk not used for recording, where they are allowed to come into contact with the disk surface

6 Why Disks Are Bit-Serial  Bit stored in an approximate trapezoid whose sides are 0.3  and 7   High TPI value makes parallel access disks unworkable Reading heads move as rigid unit Reading heads move as rigid unit One of them (the one over the servo disk) supposedly defines radial (track) position One of them (the one over the servo disk) supposedly defines radial (track) position  Ensemble is not truly rigid, and various reasons (like thermal dilations) prevent all heads being positioned over same track simultaneously, repeatedly, and reliably  Only one head is positioned accurately at a time: servo guides the assembly to approximate position of requested track, reading head does final positioning using a high frequency signal between data tracks  Sector ID always contains track number for confirmation Portable disks use shock sensors to prevent overwriting of adjacent tracks caused by jarring of R/W head Portable disks use shock sensors to prevent overwriting of adjacent tracks caused by jarring of R/W head

7 Track Densities  Until recently, only innermost track was recorded with maximum density All other tracks contained same number of sectors and bytes All other tracks contained same number of sectors and bytes  Recently, manufacturers are using bands of tracks Total number of tracks (100s-1000s) divided into bands or zones (4, 8, 16, …) each containing the same number of tracks Total number of tracks (100s-1000s) divided into bands or zones (4, 8, 16, …) each containing the same number of tracks Each band has innermost track recorded at maximum density, with other tracks having same capacity Each band has innermost track recorded at maximum density, with other tracks having same capacity Greatest capacity gains occur with a small number of bands Greatest capacity gains occur with a small number of bands  Two scheduling options Constant rotational speed, use buffer for speed matching Constant rotational speed, use buffer for speed matching Constant data rate, head spinning with rotational speed corresponding to recording density of zone Constant data rate, head spinning with rotational speed corresponding to recording density of zone

8 Sector Format  Sector is the smallest unit of data that can be read or written Typically between 32B and 4KB, with 512B being a common size Typically between 32B and 4KB, with 512B being a common size  Format of sector ID: Identifies sector with information such as angular position and track # ID: Identifies sector with information such as angular position and track #  Has its own error correcting code (ECC) GAP: Permits electronics to process ECC information GAP: Permits electronics to process ECC information DATA and its ECC DATA and its ECC IDECC GAP DATAECC GAP Total sector size is measure of formatted capacity of disk as fraction of nominal capacity

9 Disk Miscellanea  Smaller disks tend to be more cost-effective Smaller inertia, lower power consumption per megabyte, shorter seek distances, less vibration, less heat generated Smaller inertia, lower power consumption per megabyte, shorter seek distances, less vibration, less heat generated Heat generation proportional to N  RPM 2.8  D 4.6 Heat generation proportional to N  RPM 2.8  D 4.6  N = number of platters  D = diameter  RPM = rotational speed  Disk access time = seek time + rotational latency + transfer time Typical values: ms seek time, 8.3 ms rotational latency Typical values: ms seek time, 8.3 ms rotational latency  Disk growth rates Disk areal density doubling every three years Disk areal density doubling every three years Disk transfer rate doubling every five years Disk transfer rate doubling every five years Disk access time halving every ten years Disk access time halving every ten years

10 Ensembles of Disks  Key idea Use collection of disk drives to improve characteristics of disk systems (storage capacity, bandwidth, etc.) Use collection of disk drives to improve characteristics of disk systems (storage capacity, bandwidth, etc.)  Used in mainframes for a long time  RAID Redundant Array of Inexpensive Disks (original 1988 acronym) Redundant Array of Inexpensive Disks (original 1988 acronym) Redundant Array of Independent Disks (redefined in 1992) Redundant Array of Independent Disks (redefined in 1992)

11 Improving Bandwidth with Disk Arrays  Arrays of independent disk drives Similar to high-order interleaving in main memories Similar to high-order interleaving in main memories Each file assigned to different disk drive Each file assigned to different disk drive Simultaneous access to files Simultaneous access to files Load balancing issues Load balancing issues  File striping/disk striping/disk interleaving Single file distributed across array of disks Single file distributed across array of disks Similar to low-order interleaving in main memories Similar to low-order interleaving in main memories Each logical I/O request corresponds to a data stripe Each logical I/O request corresponds to a data stripe  Data stripe divided into number of equal sized stripe units  Stripe units assigned to different disk units Two kinds of striping depending on size of stripe unit Two kinds of striping depending on size of stripe unit  Fine-grained striping: Stripe unit chosen to balance load  Coarse-grained striping: Larger stripe unit

12 Improving Availability with Disk Arrays  MTTF of large-system disks approaches 1,000,000 hours  MTTF of PC-class disks approaches 150,000 hours  However, array of 1,000 PC-class disks has MTTF of 150 hours  All schemes to cope with low MTTF aim to “fail soft” Operation should be able to continue while repair is made Operation should be able to continue while repair is made Always depends on some form of redundancy Always depends on some form of redundancy