1. Chapter 12-1 Mass-Storage Systems

2. 12.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Chapter 12: Mass-Storage Systems Overview of Mass Storage Structure – will spent lots of time here. Disk Attachment Disk Scheduling Disk Management Swap-Space Management RAID Structure Disk Attachment Stable-Storage Implementation Tertiary Storage Devices Operating System Issues Performance Issues

3. 12.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Overview of Mass Storage Structures

4. 12.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Limited Objectives We can view a file system as possessing three components: A user / programmer interface to the file system The internal data structures and algorithms used by the operating system to implement this interface and access the data, and The secondary and tertiary storage structures themselves, which will be covered a couple of lectures in the future. Here we first describe the physical structure of secondary storage devices and the resulting effects on the uses of these devices

5. 12.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Overview of Mass Storage Structure Magnetic disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 200 times per second Transfer rate is rate at which data flow between drive and computer Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational delay; latency) Disk consists of a central spindle with platters attached. Data is recorded on the top and bottom surface of each platter except the top surface of the top platter and the bottom surface of the bottom platter (for dust). The read/write heads ‘float’ over the surface of the platters and all arms move with the arm assembly together in unison. The set of tracks that are ‘carved out’ via each arm position forms a cylinder.. Each track may contain hundreds of sectors, depending on the size of the sectors. Modern disks have thousands of cylinders. See next slide.

6. 12.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Moving-head Disk Machanism Discuss

7. 12.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Access The disk spins at a high speed – somewhere between 60 and 200 revolutions per second, but these speeds vary with time as technologies are constantly changing… A disk read traditionally consists of three components 1. Seek time – this is the movement of the arm to the correct cylinder 2. Head select - negligible 3. Rotational delay (latency) – generally, half the speed of rotation – until the desired sector / logical block moves under the read/write head. 4. Data transfer time - copying the data from the disk into the I/O controller unit. Oftentimes, head select is not counted, because it is done electronically. But the head must be selected so that it is decided which head is going to read/write which surface!

8. 12.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Head Crashes The read/write heads float over a surface, as mentioned. But a head crash can result from disk head making contact with the disk surface. This will ruin your entire day although it is much more unlikely nowadays. This can happen if power is abruptly pulled, although, again, more modern devices store some power so that they can gracefully degrade… Some disks are removable that this allows other disks (disk packs) to be mounted on the same disk drive. Some are ‘permanent’ disks in an organization. These are generally faster and have more capacity. Floppy Disks – inexpensive, removable magnetic disks where the head sits directly on the disk surface. Floppies rotate much more slowly and have much less capacity than hard disks. Nominal capacity of a 3.5” floppy is 1.4 MB or 2.8MB.

9. 12.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts More on Disks Disk Drives are attached to computer via I/O buses. Buses are the vehicle that support data transfer and are driven by special processors called I/O Disk Controllers generally at either end of the bus. A Host controller is located at the computer end of the bus; disk controller on the other end. In between – generally some kind of ribbon cable of varying channels. The host controller uses the bus to talk to a specific disk controller built into a disk ‘drive’ itself (or storage array – later this chapter). The computer places a specific read or write I/O command into the host controller, typically using memory-mapped I/O ports, which also points to the area of memory from which / to which data is to be accessed.. The host controller then sends the I/O command to a disk controller. The disk controller operates the disk drive hardware to accommodate the commands that this disk controller executes.  Note: these are usually called ‘commands’ vice ‘instructions.’ Much tradition here…

10. 12.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Controller Commands – and more In IBM jargon, we used to use the language: CSW – channel status word CCW – a channel command word CAW – a channel address word. These were 64 bit words executed interpretively by the disk controllers. They contain many fields of information including memory addresses, status information, specific commands (read, write, …) and much more. The interested student is encouraged to look these up. They are very interesting to see the formats of these commands. Disk controllers will typically have a built in cache to facilitate disk transfers both to the disk itself and from the disk to the host:  data transfer from the cache to the disk surface and  data transfer from the cache to the host – depending on whether we are reading or writing. Important to note that disk controllers are themselves small computers (mentioned in the past) that possess very specialized hardware to be able to interpret the commands sent to it via a limited instruction set.

11. 12.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Bus Architectures Typical buses that are available include 1. an (enhanced) integrated drive electronics (EIDE) bus, 2. an advanced technology attachment (ATA) bus, 3. a serial ATA (SATA) bus, 4. SCSI buses 5. a universal serial bus (USB) bus, a 6. fiber channel (FC) bus and Let’s look at some of the details of these at this time before we continue…

12. 12.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts EIDE Bus Architecture EIDE (Enhanced Integrated Drive Electronics) is the current standard for inexpensive, high performance hard disks used in PCs. EIDE stands for Enhanced IDE and it is a registered name own by hard disk manufacturer Western Digital. They also own the name "IDE". IDE is older technology and is pretty limited. Other companies like Seagate, IBM, Quantum and Maxtor use the term ATA, which stands for Advanced Technology Attachment. But it is all the same. (There are, however, different protocols behind the terms. ) You can think of EIDE as a bus - which has a host controller - which controls the bus, to which one may connect up to four units. All Pentium system boards since 1995 have this EIDE host controller built into the chip set.chip set This allows the hard disk and other EIDE units to be connected directly to the system board

13. 12.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts EIDE and SCSI disks Most modern computers automatically come with EIDE (enhanced IDE) built into the main board. This is perfectly adequate for personal workstations. A high performance SCSI controller can be added to a new system for an extra couple of hundred dollars. These provide for much higher performance and capability. IDE and SCSI disks operate at the same speed, but SCSI has advantages for a multitasking server because it allows many devices to be performing operations at the same time. In particular, a group of attached disks may all be transmitting / receiving at the same time thus significantly improving overall performance. This is particularly important in architectures such as RAID – ahead.

14. 12.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts EIDE and SCSI disks –The Two Main Technologies. IDE, as mentioned, is older technology and is very limited. IDE can only support disks, while EIDE supports a variety of devices IDE can only support two devices (single cable), while EIDE can support four devices as mentioned. (two cables). SCSI (Small Computer System Interface) provides a standard interface for all types of computers. The IDE disk and the ISA bus are peculiar to IBM-compatible Intel-compatible PCs. SCSI, however, is much more versatile, and is used by Macintosh computers, RISC workstations, minicomputers, and even some mainframes. SCSI has always supported a mixture of disks, tapes, and CD-ROM drives. EIDE disks are limited in size when compared to the capacity of SCSI disks. On a SCSI bus, each device is a "peer" of the other devices.

15. 12.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts More on Buses A computer is full of busses -- highways that take information and power from on­e place to another. Example: when you plug an MP3 player or digital camera into your computer, you're probably using a universal serial bus (USB) (ahead) port. Your USB port is good at carrying the data and electricity required for small electronic devices that do things like create and store pictures and music files. But this bus isn't big enough to support a whole computer, a server or lots of devices simultaneously. (more ahead on USB devices/ports) To support a number of devices, we would need something like a SCSI. SCSI originally stood for Small Computer System Interface, but it's really outgrown the "small" designation. SCSI a fast bus that can connect lots of devices to a computer at the same time, including hard drives, scanners, CD-ROM/RW drives, printers, and tape drives.

16. 12.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts SCSI is an ultra-fast, high-power communications bus that connects up to 15 devices to your computer.

17. 12.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Control Card and Connector SCSI devices usually connect to a controller card like this one

18. 12.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts EIDE and SCSI disks –The Two Main Technologies. An IDE disk must be mounted inside the computer. There is no provision for the IDE ribbon cable to run to external devices. Internal SCSI: SCSI devices can also be internal. They are connected to each other and to the adapter card using a flat ribbon cable or a round bundled cable.. External SCSI: SCSI devices, however, can also be external to the computer. They can be mounted in individual boxes, or can be mounted together in larger tower enclosures. This makes SCSI devices much more flexible to a variety of architectures, as we shall see ahead.

19. 12.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts USB Serial Bus In information technology, a Universal Serial Bus (USB) is a serial bus standard that interfaces many different kinds of devices to a host computer. USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve the plug and play capabilities by allowing hot swapping; that is, by allowing devices to be connected and disconnected without rebooting the computer or turning off the device. We nowadays do this all the time – plugging in and removing external devices via our USB ports. Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed Consider our jump drives, ipods, and a host of other devices.

20. 12.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts USB Serial Bus USB is intended to help retire all legacy varieties of serial and parallel ports. USB can connect many computer peripherals such as mice, keyboards, PDAs, joysticks, scanners, digital cameras, printers, flash drives and more! For many such devices, USB has become the standard connection method. USB was originally designed for personal computers. But it has become commonplace on other devices such as PDAs and video game consoles, and as a bridging power cord between a device and an AC adapter plugged into a wall plug for charging purposes. As of 2008, there are about 2 billion USB devices in the world. Interesting, other technologies, like serial-ATA (SATA), are largely replacing USBs in new systems, but SCSI is still in use.

21. 12.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts The most common USB Plug.

22. 12.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Fiber Channel Bus (FC) Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the International Committee for Information Technology Standards (INCITS), an American National Standards Institute (ANSI)–accredited standards committee. It started use primarily in the supercomputer field, but has become the standard connection type for storage area networks (SAN) in enterprise storage. We are going to discuss storage area networks later in this chapter. Despite common connotations of its name, Fibre Channel signaling can run on both  twisted pair copper wire and  fiber-optic cables. Fibre Channel Protocol (FCP) is a transport protocol (similar to TCP used in IP networks) which predominantly transports SCSI commands over Fibre Channel networks.

23. 12.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Magnetic Tapes Was the primary early secondary-storage medium for many years. Relatively permanent and holds large quantities of data Access time slow, but again, can store huge quantities of data. Random access ~1000 times slower than disk Mainly used for backup, storage of infrequently-used data, transfer medium between systems Used for archiving; history tapes, and more. Please note that in years past, these constituted a primary storage medium for files – as long as they were sequential!! Kept in spools and wound or rewound past read-write head Once data under head, transfer rates comparable to disk 20-200GB typical storage

24. 12.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Used extensively in the mid-60s and early 70s. Still – backup / recovery… Tapes are still widely used.

25. 12.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Structure

26. 12.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Structure Unlike years ago, disks are structured differently with different technologies. Two major types of technologies: Have lots of similarities, but Have major differences too. They are: Constant linear velocity (CLV) disks and Constant angular velocity (CAV) disks. Their organizations / way they store data is different.

27. 12.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Structure – Commonalities Disks store data as large, one-dimensional arrays of blocks. The term “logical blocks” is now used in place of what used to be called (and definitely still is in some sectors) “physical blocks.” We will go with the more modern terminology, but be careful. We will distinguish where appropriate. Most common logical block size is 512 bytes (.5 K) Other sizes available for low level formatting. Sector 0 is traditionally the first sector on the first track of the outermost cylinder and mapping proceeds from this sector, this track, this cylinder to the rest of the tracks on that cylinder before moving to the next cylinder. We can convert a logical block into the old-style disk address that address a cylinder number, a track number in that cylinder, and a record / sector number on that track.

28. 12.28 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Structure – Commonalities But life is not simple and we cannot always do a direct mapping; that is, we don’t always have sector 0, sector 1, etc. through tracks, cylinders, etc. without a hitch! Manufacturing of disks usually includes defective sectors (or even tracks). They come from the manufacturer that way and defects are normally identified via low-level formatting normally done at the factory. Then too, the number of sectors per track may not be a constant on some kind of drives.

29. 12.29 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Constant Linear Velocity (CLV) Drives CD-ROM and DVD-ROM drives use the CLV approach. This is referred to as a constant linear velocity drive. Here, the density of each track is uniform. This means that the same amount of storage is available on each track no matter where the track is on the disk. Now, as tracks are located away from the center of the disk, they are clearly ‘longer’ (if we opened them up and spread them out) than other tracks. Result is that there are more sectors per track as we move outward, but the density of the bits is uniform. Because of this, (your textbook) tracks in the outermost cylinders can hold 40% more data (additional sectors) than tracks in the inner cylinders! The same rate of data is moving under the read/write heads to keep transfer speeds uniform.

30. 12.30 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Constant Linear Velocity drives - more An interesting twist to this: Constant Linear Velocity Storage (CLV) is a driving scheme in which the linear velocity of the disk is kept constant. This requires that the angular velocity of the disk be larger when the reading or writing tracks closer to the axis. This is necessary so that the same number of bits pass under a read/write head per unit time. The advantage of this technique is that the read/write speed is constant. Downside: But, as mechanical stability puts an upper limit on the angular velocity (and not the linear velocity) using the same linear velocity throughout, means that we are using less than the maximal angular velocity at outer tracks and this means that full potential of the drive is not used. Lots of devices use CLV drives. These are necessary to keep a constant data rate.. Interestingly, mechanically, the motor speed actually decreases from 495 to 212 rpm as the read head moves away from the center, to keep the disc moving past the read head at a CLV of 1.2 meters/sec.

31. 12.31 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Constant Angular Velocity (CAV) drives In contrast to the constant linear velocity (CLV) drives, we have the constant angular velocity (CAV) drives. Here, unlike the CLV drives, the disk rotation speed stays constant, but as a result, the density of the bits moving under the read/write heads decreases as we proceed from inner tracks to outer tracks. But since the disk rotation speed stays constant, the data transfer rate remains constant. Newer Technologies: As technology has progressed, the number of sectors per track has been increasing, and now the outermost tracks often have several hundred sectors per track. Further, the number of cylinders / disk is on the upswing. Large disks have tens of thousands of cylinders!

32. 12.32 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Constant Angular Velocity drives – more A drive or disc operating in CAV mode maintains a constant angular velocity, contrasted with a constant linear velocity (CLV). When playing back music, a compact disc (CD) employs CLV to maintain a constant data rate. As mentioned, the motor speed decreases from 495 to 212 rpm as the read head moves away from the center to keep the disc moving past the read head at a constant linear velocity. CAV was used in the LaserDisc format for interactive titles, as well as special editions of certain films. CAV allowed for perfect still frames, as well as random access to any given frame on a disc. Playing time, however, was cut in half from 60 minutes to 30 minutes. More on CAVs: High speed CD and DVD drives use CAV. CAV is used with Nintendo GameCube Game Disc and Wii Optical Disc. To accommodate the higher data transfer rates and random access requirements of modern CD-ROM drives, CAV systems are used. This is because seek performance would be greatly affected during random access by the requirement to continually modulate the disc's rotation speed to be appropriate for the read head's position.

33. End of Chapter 12.1