Mass-Storage Structure Operating System Concepts chapter 12 CS 355 Operating Systems Dr. Matthew Wright

Background: Magnetic Disks Rotate 60 to 200 times per second Transfer rate: rate at which data flows between drive and computer Positioning time (random-access time): time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)

Disk Address Structure Disks are addressed as a large 1-dimensional array of logical blocks (usually 512 bytes per logical block). This array is mapped onto the sectors of the disk, usually with sector 0 on the outermost cylinder, then through that track, then through that cylinder, and then through the other cylinders working toward the center of the disk. Converting logical addresses to cylinder and track numbers is difficult because: Most disks have some defective sectors, which are replaced by spare sectors elsewhere on the disk. The number of sectors per track might not be constant. Constant linear velocity (CLV): tracks farther from center hold more bits, so disk rotates faster when reading these tracks to keep data rate constant (CSs, DVDs commonly use this method) Constant angular velocity (CAV): rotational speed is constant, so bit density decreases from inner tracks to outer tracks to keep data rate constant

Disk Scheduling: FCFS Simple, but generally doesn’t provide the fastest service Example: suppose the read/write heads start on cylinder 53, and disk queue has requests for I/O to blocks on the following cylinders: 98, 183, 37, 122, 14, 124, 65, 67 Diagram shows read/write head movement to service the requests FCFS. Total head movement spans 640 cylinders.

Disk Scheduling: SSTF Shortest Seek Time First (SSTF): service the requests closest to the current position of the read/write heads This is similar to SJF scheduling, and could starve some requests. Example: heads at cylinder 53; disk request queue contains: 98, 183, 37, 122, 14, 124, 65, 67 Diagram shows read/write head movement to service the requests SSTF. Total head movement spans 236 cylinders.

Disk Scheduling: SCAN SCAN algorithm: disk heads start at one end, move towards the other end, then return, servicing requests along each way Example: heads at cylinder 53 moving toward 0; request queue: 98, 183, 37, 122, 14, 124, 65, 67 Diagram shows read/write head movement to service the requests with SCAN algorithm. Total head movement spans 236 cylinders.

Disk Scheduling: C-SCAN Circular SCAN (C-SCAN): Disk heads start at one end, move towards the other end, servicing requests along each way. Disk heads return immediately to the first end without servicing requests, then repeat. Example: heads at cylinder 53; request queue: 98, 183, 37, 122, 14, 124, 65, 67 Diagram shows read/write head movement to service the requests with C- SCAN algorithm. Total head movement spans 383 cylinders.

Disk Scheduling: LOOK and C-LOOK Like SPAN or C-SPAN algorithms, but only going as far as the last request in either direction. Example: heads at cylinder 53; request queue: 98, 183, 37, 122, 14, 124, 65, 67 Diagram shows read/write head movement to service the requests with C- SCAN algorithm. Total head movement spans 322 cylinders.

Selecting a Disk-Scheduling Algorithm Which algorithm to choose? SSTF is common and better than FCFS. 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, and the file- allocation method. In general, either SSTF or LOOK is a reasonable choice for the default algorithm. 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. Why not let the controller built into the disk hardware manage the scheduling? The disk hardware can take into account both seek time and rotational latency. The OS may choose to mandate the disk scheduling to guarantee priority of certain types of I/O.

Disk Management The Operating System may also be responsible for tasks such as disk formatting, booting from disk, and bad-block recovery. Low-level formatting divides a disk into sectors, and is usually performed when the disk is manufactured. Logical formatting creates a file system on the disk, and is done by the OS. The OS maintains the boot blocks (or boot partition) that contain the bootstrap loader. Bad blocks: disk blocks may fail An error-correcting code (ECC) stored with each block can detect and possibly correct an error (if so, it is called a soft error). Disks contain spare sectors which are substituted for bad sectors. If the system cannot recover from the error, it is called a hard error, and manual intervention may be required.

Swap-Space Management Recall that memory uses disk space as an extension of main memory; this disk space is called the swap space, even for systems that implement paging rather than pure swapping. Swap-space can be: A file in the normal file system: easy to implement, but slow in practice A separate (raw) disk partition: requires a swap-space manager, but can be optimized for speed rather than storage efficiency Linux allows the administrator to choose whether the swap space is in a file or in a raw disk partition.

RAID Structure RAID: Redundant Array of Independent Disks or Redundant Array of Inexpensive Disks In systems with large numbers of disks, disk failures are common. Redundancy allows the recovery of data when disk(s) fail. Mirroring: A logical disk consists of two physical disks, and every write is carried out on both disks. Bit-level striping: Splits the bits of each byte across multiple disks, which improves the transfer rate. Block-level striping: Splits blocks of a file across multiple disks, which improves the access rate for large files and allows for concurrent reads of small files. A nonvolatile RAM (NVRAM) cache can be used to protect data waiting to be written in case a power failure occurs. Are disk failures really independent? What if multiple disks fail simultaneously?

RAID Levels RAID level 0: non-redundant striping Data striped at the block level, with no redundancy. RAID level 1: mirrored disks Two copies of data stored on different disks. Data not striped. Easy to recover data from one disk that fails RAID 0 + 1: combines RAID levels 0 and 1 Provides both performance and reliability.

RAID Levels RAID level 2: error-correcting codes Data striped across disks at the bit level. Disks labeled P store extra bits that can be used to reconstruct data if one disk fails. Requires fewer disks than RAID level 1. Requires computation of the error-correction bits at every write, and failure recovery requires lots of reads and computation.

RAID Levels RAID level 3: bit-interleaved parity Data striped across disks at the bit level. Since disk controllers can detect whether a sector has read correctly, a single parity bit can be used for error detection and correction. As good as RAID level 2 in practice, but less expensive. Still requires extra computation for parity bits. RAID level 4: block-interleaved parity Data striped across disks at the block level. Stores parity blocks on a separate disk, which can be used to reconstruct the blocks on a single failed disk.

RAID Levels RAID level 5: block-interleaved distributed parity Data striped across disks at the block level. Spreads data and parity blocks across all disks. Avoids possible overuse of a single parity disk, which could happen with RAID level 4. RAID level 6: P + Q redundancy scheme Like RAID level 5, but stores extra redundant information to guard against simultaneous failures of multiple disks. Uses error-correcting codes such as Reed-Solomon codes.

RAID Implementation RAID can be implemented at various levels: At the kernel of system software level By the host bus-adapter hardware By storage array hardware In the Storage Area Network (SAN) by disk virtualization devices Some RAID implementations include a hot spare: an extra disk that is not used until one disk fails, at which time the system automatically restores data onto the spare disk.

Stable-Storage Implementation Stable storage: storage that never loses stored information. Write-ahead logging (used to implement atomic transactions) 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 failure during an update will not leave all copies in a damaged state, and so that we can safely recover from a failure. Three possible outcomes of a disk write: Successful completion: all of the data written successfully Partial failure: only some of the data written successfully Total failure: occurs before write starts; previous data remains intact

Stable-Storage Implementation Strategy: maintain two (identical) physical blocks for each logical block, on different disks, with error-detection bits for each block A write operation proceeds as: Write the information to the first physical block. When the first write completes, then write the same information to the second physical block. When the second write completes, then declare the operation successful. During failure recovery, examine each pair of physical blocks: If both are the same and neither contains a detectable error, then do nothing. If one block contains a detectable error, then replace its contents with the other block. If neither block contains a detectable error, but the values differ, then replace the contents of the first block with that of the second. As long as both copies don’t fail simultaneously, we guarantee that a write operation will either succeed completely or result in no change.

Tertiary Storage Most OSs handle removable disks almost exactly like fixed disks — a new cartridge is formatted and an empty file system is generated on the disk. Tapes are presented as a raw storage medium, i.e., and application does not open a file on the tape, it opens the whole tape drive as a raw device. Usually the tape drive is reserved for the exclusive use of that application. Since the OS does not provide file system services, the application must decide how to use the array of blocks. Since every application makes up its own rules for how to organize a tape, a tape full of data can generally only be used by the program that created it. The issue of naming files on removable media is especially difficult when we want to write data on a removable cartridge on one computer, and then use the cartridge in another computer. Contemporary OSs generally leave the name space problem unsolved for removable media, and depend on applications and users to figure out how to access and interpret the data. Some kinds of removable media (e.g., CDs) are so well standardized that all computers use them the same way.