CSE325 Principles of Operating Systems. Mass-Storage Systems. David P. Duggan. April 19, 2011
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1 CSE325 Principles of Operating Systems Mass-Storage Systems David P. Duggan April 19, 2011
2 Outline Storage Devices Disk Scheduling FCFS SSTF SCAN, C-SCAN LOOK, C-LOOK Redundant Arrays of Inexpensive Disks RAID 0-6 4/21/11 CSE325 - Storage 2
3 Devices: Magnetic Disks Purpose Long-term, nonvolatile storage Large, inexpensive, slow level in the storage hierarchy Characteristics Seek Time positional latency rotational latency Rotational rate 60 to 200 times per second Capacity Terabytes Quadruples every 3 years 3
4 Moving-head Disk Mechanism 4/21/11 4
5 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. 4/21/11 CSE325 - Storage 5
6 Magnetic Tapes Much slower than magnetic disk in positioning Comparable speeds when reading and writing Capacity from 20GB to 200GB Long term storage 4/21/11 CSE325 - Storage 6
7 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 iscsi protocol uses IP network to carry the SCSI protocol Performance problems? 4/21/11 7
8 Storage Area Network Common in large storage environments (and becoming more common) Multiple hosts attached to multiple storage arrays - flexible 4/21/11 CSE325 - Storage 8
9 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 for the disk 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 seek time Seek time 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. 4/21/11 CSE325 - Storage 9
10 Disk Scheduling (Cont.) Several algorithms exist to schedule the servicing of disk I/O requests. We illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53 4/21/11 CSE325 - Storage 10
11 FCFS Illustration shows total head movement of 640 cylinders." Can you develop a better disk scheduler? 4/21/11 11
12 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. Can you further improve the performance? 4/21/11 12
13 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. 4/21/11 CSE325 - Storage 13
14 SCAN (Cont.) Head movement of 236 cylinders. 4/21/11 CSE325 - Storage 14
15 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. 4/21/11 CSE325 - Storage 15
16 C-SCAN (Cont.) Is there a problem here? 4/21/11 CSE325 - Storage 16
17 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. 4/21/11 CSE325 - Storage 17
18 C-LOOK (Cont.) 4/21/11 CSE325 - Storage 18
19 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 fileallocation 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. 4/21/11 CSE325 - Storage 19
20 Disk Management Disk initialization Booting from disk Bad blocks 4/21/11 CSE325 - Storage 20
21 Disk Initialization Formatting Low-level or physical Usually done at factory now Sectors contain ECC for fixing errors Partitioning Break disk into groups of cylinders (logical disks) Formatting Creating filesystem 4/21/11 CSE325 - Storage 21
22 Booting from Disk Boot block Bootstrap code finds and loads OS kernel Uses boot partition Boot sequence/process Read Master-Boot-Record Located in first sector on bootable disk Directs load to come from first sector of boot partition 4/21/11 CSE325 - Storage 22
23 Bad Blocks Disks can fail! Poor manufacturing Tight tolerances Plain old use Sector sparing Spare sectors used when OS or disk finds bad blocks Could invalidate efficiencies OS has made Sector slipping Slide sectors down to first free spare sector 4/21/11 CSE325 - Storage 23
24 Swap Space Management Use Overestimate rather than underestimate Separate disks spreads out I/O load Location In filesystem Separate partition Decisions When to use swap space? 4/21/11 CSE325 - Storage 24
25 Redundant Arrays of (Inexpensive) Disks Files are "striped" across multiple disks Redundancy yields high data availability Availability: service still provided to user, even if some components failed Disks will still fail Contents reconstructed from data redundantly stored in the array Capacity penalty to store redundant info Bandwidth penalty to update redundant info 4/21/11 CSE325 - Storage 25
26 RAID Levels 4/21/11 CSE325 - Storage 26
27 Redundant Arrays of Inexpensive Disks RAID 1: Disk Mirroring/Shadowing recovery! group! Each disk is fully duplicated onto its mirror Very high availability can be achieved Bandwidth sacrifice on write: Logical write = two physical writes Reads may be optimized (How?) Most expensive solution: 100% capacity overhead RAID 2: Disks are synchronized and striped in single bytes/words; Hamming code parity. 4/21/11 27
28 RAID 3: Bit-Interleaved Parity Organization Parity Disk ! ! !.. logical record! P! Byte-level striping P contains sum of other disks per stripe mod 2 ( parity ) If disk fails, subtract P from sum of other disks to find missing information Bit Interleaved Parity 28
29 RAID 3 Sum computed across recovery group to protect against hard disk failures, stored in P disk Logically, a single high capacity, high transfer rate disk: good for large transfers 33% capacity cost for parity if 3 data disks and 1 parity disk Wider arrays reduce capacity costs, but decreases availability 4/21/11 CSE325 - Storage 29
30 Inspiration for RAID 4 Allows independent reads to different disks simultaneously Block-level parity with parity on a separate disk from all data blocks Can add new disks by initializing data to 0 4/21/11 CSE325 - Storage 30
31 RAID 4 Block-Interleaved Parity Organization High I/O Rate Parity Insides of 5 disks! D0! D1! D2! D3! P! D4! D5! D6! D7! P! Increasing" Logical" Disk " Address" D8! D9! P! D10! D11! Example:! small read D0 & D5, large write D12-D15! Block Interleaved Parity D12! P! D13! D14! D15! D16! D17! D18! D19! P! D20! D21! D22! D23! P! Disk Columns! Stripe! 31
32 Inspiration for RAID 5 RAID 4 works well for large reads Small writes (write to one disk): Option 1: read other data disks, create new sum and write to Parity Disk Option 2: since P has old sum, compare old data to new data, add the difference to P Small writes are limited by Parity Disk: Write to D0, D5 both also write to P disk D0! D1! D2! D3! P! D4! D5! D6! D7! P! 4/21/11 32
33 RAID 5: Block-Interleaved Distributed Parity High I/O Rate Interleaved Parity Independent writes! possible because of! interleaved parity! Example: write to D0, D5 uses disks 0, 1, 3, 4! D0! D1! D2! D3! P! D4! D5! D6! P! D7! D8! D9! P! D10! D11! D12! P! D13! D14! D15! P! D16! D17! D18! D19! D20! D21! D22! D23! P! Disk Columns! Increasing! Logical! Disk! Addresses! 33
34 Problems of Disk Arrays Small Writes RAID-5: Small Write Algorithm 1 Logical Write = 2 Physical Reads + 2 Physical Writes D0' D0 D1 D2 D3 P new data old data (1. Read) old parity (2. Read) + XOR + XOR (3. Write) (4. Write) D0' D1 D2 D3 P' 34
35 RAID Levels 4/21/11 CSE325 - Storage 35
36 RAID (0 + 1) and (1 + 0) 4/21/11 CSE325 - Storage 36
37 Operating System Issues Major OS jobs are to manage physical devices and to present a virtual machine abstraction to applications For hard disks, the OS provides two abstraction: Raw device an array of data blocks. File system the OS queues and schedules the interleaved requests from several applications. 4/21/11 CSE325 - Storage 37
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