12.1 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Chapter 10: Mass-Storage Structure Overview of Mass Storage Structure Disk Structure Disk Scheduling Disk Management Swap-Space Management RAID Structure Stable-Storage Implementation Tertiary Storage Devices Operating System Issues Performance Issues

12.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Objectives Describe the physical structure of secondary and tertiary storage devices and the resulting effects on the uses of the devices Explain the performance characteristics of mass-storage devices Discuss operating-system services provided for mass storage, including RAID (Redundant Arrays of Inexpensive Disks) and HSM (Hierarchical Storage Management)

12.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Overview of Mass Storage Structure Magnetic disks provide bulk of secondary storage of modern computers Most drives rotate at 60 to 200 times per second Transfer rate is rate at which data flows between drive and computer  As of 2010, a typical 7200 rpm desktop hard drive has a sustained disk-to-buffer transfer rate of upto 1030 Mbits/sec Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) (Seektime is about 9ms for current desktops.) and time for desired sector to rotate under the disk head (rotational latency). Head crash results from disk head making contact with the disk surface Disks can be removable Drive attached to computer via I/O bus Busses vary, including enhanced integrated drive electronics (EIDE), advanced technology attachment (ATA), Serial ATA (SATA 3Gbits/sec), Universal serial bus (USB), Fibre Channel (FC), Small computer system interface (SCSI), Serial attached SCSI (SAS), Enhanced Small disk interface(ESDI),

12.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Overview of Mass Storage Structure … Solid State Disks (SSDs): It is non-volatile memory that is used as disk. There are many variations. Could be DRAM with a battery for maintaining its state in a power failure through flash-memory technologies. Advantages and disadvantages of SSDs  Could be more reliable than disks because there are no moving parts  Are faster because they have no seek time and consume less power  More expensive than traditional hard disks.  Have less capacity than hard disk  May have shorter lifetime than hard disks.  Some systems use it as replacement of disks, others use it as new cache between memory and disk

12.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Overview of Mass Storage Structure (Cont.) Magnetic tape Was early secondary-storage medium Relatively permanent and holds large quantities of data Access time slow Random access ~1000 times slower than disk Mainly used for backup, storage of infrequently-used data, transfer medium between systems Kept in spool and wound or rewound past read-write head Once data under tape head, transfer rates comparable to disk Tape capacities vary and exceed several terabytes. Common technologies are 4mm, 8mm, 19mm, LTO-2 and SDLT

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

12.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially. Sector 0 is the first sector of the first track on the outermost cylinder. Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.

12.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Network-Attached Storage Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus) NFS and CIFS are common protocols Implemented via remote procedure calls (RPCs) between host and storage

12.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Storage Area Network Common in large storage environments (and becoming more common) Multiple hosts attached to multiple storage arrays - flexible

12.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Scheduling The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth. Access time has two major components Seek time is the time to move the heads to the cylinder containing the desired sector. Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. Minimize average seek time – Any suggestions on how to do this? Seek time is proportional to seek distance Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer.

12.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Scheduling (Cont.) Several algorithms exist to schedule the servicing of disk I/O requests. We illustrate them with a request queue (0-199 cylinders). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53

12.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts FCFS Illustration shows total head movement of 640 cylinders.

12.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts SSTF Selects the request with the minimum seek time from the current head position. SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests. Illustration shows total head movement of 236 cylinders.

12.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts SSTF (Cont.)

12.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts SCAN The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. Sometimes called the elevator algorithm. Illustration shows total head movement of 208 cylinders.

12.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts SCAN (Cont.)

12.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts C-SCAN Provides a more uniform wait time than SCAN. The head moves from one end of the disk to the other, servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.

12.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts C-SCAN (Cont.)

12.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts C-LOOK Version of C-SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk.

12.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts C-LOOK (Cont.)

12.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Selecting a Disk-Scheduling Algorithm SSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that place a heavy load on the disk. Performance depends on the number and types of requests. Requests for disk service can be influenced by the file-allocation method. The disk-scheduling algorithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary. Either SSTF or C-LOOK is a reasonable choice for the default algorithm.

12.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Disk Management Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write. To use a disk to hold files, the operating system still needs to record its own data structures on the disk. Partition the disk into one or more groups of cylinders. Logical formatting or “making a file system”. Boot block initializes system. The bootstrap is stored in ROM. Bootstrap loader program. Methods such as sector sparing or sector slipping is used to handle bad blocks. Under sector sparing, a new sector is allocated to replace a bad sector. Under sector slipping: for example, if sector 20 becomes defective, and the first available sector follows sector 202, then all sectors from 20 to 202 are remapped moving them down one spot.

12.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Swap-Space Management Swap-space — Virtual memory uses disk space as an extension of main memory. Swap-space can be carved out of the normal file system,or, more commonly, it can be in a separate disk partition. Swap-space management 4.3BSD allocates swap space when process starts; holds text segment (the program) and data segment. Kernel uses swap maps to track swap-space use. Solaris 2 allocates swap space only when a page is forced out of physical memory, not when the virtual memory page is first created.

12.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Data Structures for Swapping on Linux Systems A swap area may be a either a swap file, or a raw swap partition Each swap area consists of series of 4KB page slots, which are used to hold swap pages Associated with each swap area is a swap map – an array of integer counters, each corresponding to a page slot in the swap area. If the value of the counter is 0, the corresponding page slot is available; if it is greater than 0, the number indicates the number of processes to which the swapped page is mapped.

12.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts RAID Structure Improving reliability via redundancy. Suppose the mean time to failure of a disk is 100,000 hours (more than 10 years). Then the meantime to failure of a single disk in an array of 100 disks will be /100 = 1000 hours (41.66 days). If only one copy of the data is stored, then each disk failure will cause the data on that disk to be lost. A solution to this problem is introducing redundancy RAID (Redundant array of Inexpensive (now Independent) Disks – multiple disk drives provide reliability via redundancy. Innovators of RAID: David Patterson, Randy Katz and Garth Gibson RAID is arranged into six different levels.

12.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts RAID (cont) Several improvements in disk-use techniques involve the use of multiple disks working cooperatively. Disk striping uses a group of disks as one storage unit. RAID schemes improve performance and improve the reliability of the storage system by storing redundant data. Mirroring or shadowing keeps duplicate of each disk.  If the mean time to failure of a single disk is 100,000 hours and the mean time to repair is 10 hours, the mean time to data loss of a mirrored disk system is (100,000*100,000)/(2*10) hours (= 57,000 years). Block interleaved parity uses much less redundancy.

12.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts RAID Levels

12.28 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Various RAID levels RAID 0: non-redundant striping. Refers to disk arrays with striping at the level of blocks without any redundancy (No fault tolerance). It only helps in increasing the disk bandwidth. RAID 1: Disk mirroring. RAID 2: Memory-style error-correcting code organization. Uses error correcting codes to correct errors. Usually should tolerate more than one failure, but the book says it can tolerate only one failure. RAID 3: Bit-interleaved parity organization. Based on the idea that a single parity bit can be used for error detection as well error correction. If one of the sectors is damaged, we can figure out all the bits in the sector by computing the parity of the corresponding bits in the respective sectors of the other disks. RAID 4: Block interleaved parity organization. Uses block level striping, as in RAID 0 and in addition keeps a parity block on a separate disk for corresponding blocks in N other disks. If one of the disks fail, the parity block along with the corresponding blocks on the other disks can be used to restore the blocks on the failed disk.

12.29 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Various RAID levels … RAID 5: uses block interleaved distributed parity. Differs form RAID 4 by spreading data and parity among all N+1 disks, rather than storing data on N disks and parity in one disk. For example, with an array of five disks, the parity of the nth block is stored in disk (n mod 5) + 1; the nth blocks of the other four disks store the actual data for that block. RAID 6: also called P+Q redundancy scheme, uses error- correcting codes such as Reed-Solomon code to guard against multiple disk failures.

12.30 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts RAID (0 + 1) and (1 + 0)

12.31 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Stable-Storage Implementation Write-ahead log scheme requires stable storage. To implement stable storage: Replicate information on more than one nonvolatile storage media with independent failure modes. Update information in a controlled manner to ensure that we can recover the stable data after any failure during data transfer or recovery.