Mass-Storage Structure

1. Mass-Storage Structure CS 540 – Chapter 12 Kate Dehbashi (kate_dehbashi@yahoo.com) Anna Deghdzunyan Fall 2010 Dr. Behzad

2. Agenda • Review • File System • Parts of File System • Overview • Magnetic Disks • Magnetic Tapes • Disk Structure • Disk Attachment • Host-attached • Network-attached • Storage-Area Networks • Disk Scheduling • Scheduling Algorithms • Selection of an algorithm

3. Agenda (Cont.) • Disk Management • Disk Formatting • Boot Block • Bad-Block Recovery • Swap-Space Management • How is it used • Where is it located • How is it managed

4. Agenda (Cont.) • RAID structure • RAID levels • The implementation of RAID • Problems with RAID • Stable Storage Implementation • Disk write result • Recoverable Write • Failure detection and recovery • Tertiary-Storage Structure • Tertiary-Storage Devices • Removable disks • Tapes • OS support • Tape Drives • File Naming • HSM • Speed • Reliability • Cost

5. Review • File System • Method of storing and organizing computer files and their data • Storage • Organization • Manipulation • Retrieval • Maintain physical location

6. Review (Cont.) • Parts of File System • Interface • User and programmer interface to the file system • Implementation • Internal data structure and algorithms used to implement the interface • Storage Structure • Physical structure • Disk scheduling algorithms • Disk formatting • Disk reliability • Stable-storage implementation

7. Overview • Magnetic Disks • Magnetic Tape

8. Magnetic Disks • Structure • Platter • Track • Sector • Cylinder • Disk arm • Read-write head • Rotates 60-200 times/second • Disk Speed • Transfer rate • Positioning time • Seek time • Rotational latency

9. Magnetic Disks (Cont.) • Head Crash • Disk head making contact with the disk surface • Permanent damage • Removable Magnetic Disks • Floppy • Head sits directly on the surface  Slow rotation and lower disk space • I/O bus • Drive attached to computer via set of wires • Busses vary, including EIDE, ATA, SATA, USB, Fiber Channel, SCSI, Fire wire • Disk Controller • Cache memory • Host controller

11. First HDD – IBM RAMAC 1956 1.5 square meters (16 sq ft). $3200.00

13. Magnetic Tape • Early secondary-storage medium • First used in1951 as a computer storage • Holds large quantities of data • LTO-5 (2010) 1.5 TB uncompressed data (book: 20-200GB) • Access time slow • Random access ~1000 times slower than disk • Once data under head, transfer rates comparable to disk • Modern Usage • Backup, archive • For large amount of data, tape can be substantially less expensive than disk • Common technologies • 4mm, 8mm, 19mm • LTO (Linear Tape-Open), SDLT (Digital Linear Tape)

14. LTO-2 ¼ , ½ inch SDLT

15. Disk Structure • Addressing • One-dimensional array of blocks • Logical Block • Smallest unit of transfer • 512 bytes • Blocks maps to sectors sequentially • Sector 0: first sector, first track, outmost cylinder • Mapping order • Track • Rest of the tracks in the same cylinder • Rest of the cylinders from outermost to innermost

16. Disk Structure (Cont.) • Logical block number • Cylinder#, track#, sector# • In practice it is difficult to perform • Defective sectors • Sectors/track is not constant • CLV (Constant Linear Velocity) • Constant density of bits/track • Variable rotational speed • CAV (Constant Angular Velocity) • Constant rotational speed • Variable density of bits per track

17. Disk Attachment • Host-Attached Storage (DAS) • Accessed through local I/O ports • IDE, ATA, SATA • SCSI, FC • Wide variety of storage devices • HDD, RAID Arrays, CD/DVD Drives and Tape • Network-Attached Storage (NAS) • NAS • ISCSI • Storage-Area Network (SAN) • SAN • infiniBand

18. Host-Attached StorageSCSI • SCSI (Small Computer System Interface) • Large variety of devices • 16 devices per cable • Controller card (SCSI Initiator) • SCSI target • 8 logical units per target

20. Host-Attached Storage FC • FC (Fiber Channel) • High-speed serial architecture • Optical cable, four-conductor copper cable • Switched fabric (FC - SW) • All devices are connected to fiber channel switches • 24-bit address space  multiple hosts and storage devices • Dominate in future • Basic of SANs • Arbitrated Loop (FC – AL) • 126 devices • All devices are in a loop or ring • Historically lower cost but rarely used now

21. FC – Topologies

22. Network-Attached Storage • NAS • Storage system • Accessed remotely over a data network • Clients access via remote-procedure-call interface • UNIX: NFS • Windows: CIFS • RPCs carried via TCP/UDP • Convenient way for all clients to share a pool of storage • NAS VS local-attached • Same ease of naming and access • Less efficient and lower performance

25. Network-Attached Storage (Cont.) • ISCSI – Internet Small Computing System Interface • Latest NAT protocol • IP-based storage networking protocol • Uses IP network to carry SCSI Protocol • Clients are able to send SCSI commands to remote targets • TCP ports 860 and 3260

26. Storage-Area Networks • SAN • Private network connecting servers and storage devices • Uses storage protocols instead of networking protocols • Multiple hosts and storage can attach to the same SAN  flexibility • SAN Switch allows/prohibits client access (exp.) • FC is the most common SAN interconnect

29. Storage-Area Networks (Cont.) • InfiniBand • Special-purpose bus architecture • Supports high-speed interconnection network • Up to 2.5 gbps • 64,000 addressable devices • Supports QoS and Failover

32. Disk Scheduling • Disk Drive Efficiency • Access time • Seek time • Rotational latency • Bandwidth • Bytes transferred / Δt • Δt: Completion time of the last transfer – first request for service time • Improve? • Scheduling the servicing of I/O requests in a good order

33. Disk Scheduling (Cont.) • I/O request procedure • System Call Sent by the process to the OS • System Call information • Input/output • Disk address • Memory address • Number of sectors to be transferred • If disk available  access else  Queue

34. Disk Scheduling (Cont.) • Algorithms • FCFS • SSTF • SCAN • C-SCAN • LOOK/CLOOK

35. Disk Scheduling (Cont.) • FCFS (First come First Served) 640 cylinder moves

36. Disk Scheduling (Cont.) • SSTF (Shortest Seek Time First) • Service requests close to the current head position • Starvation 236 cylinder moves

37. Disk Scheduling (Cont.) • SCAN (Elevator Alg.) • Head starts at one end and goes to the other end • Services each request on the current track 236 cylinder moves

38. Disk Scheduling (Cont.) • CSCAN • Variant of SCAN • More uniform wait time • When head reaches the end, immediately moves to the beginning without servicing any request 360 cylinder moves

39. Disk Scheduling (Cont.) • LOOK/CLOOK • Head goes as far as the last request in each direction 322 cylinder moves

40. Disk Scheduling (Cont.) • Selection of an algorithm Factors • SSTF is common and better performance than FCFS • SCAN, CSCAN perform better for systems that place a heavy load on the disk No starvation • Scheduling alg. Performance (example1) • Number of requests • Types of requests • Requests for disk service can be influenced by • The file-allocation method (example2) • Location of directories and indexed blocks • Caching directories and indexed blocks in the main memory reduces arm movement (example3)

41. Disk Scheduling (Cont.) • Selection of an algorithm • Separate module of the OS  can be replaced if necessary • Default: SSTF/LOOK • Rotational Delay Perspective • Modern disks do not disclose the physical location of logical blocks • Disk controller takes over OS to choose the alg. • Problem? • If only I/O  OK • But there are other constraints • Example: request for paging (example)

42. Disk Management • Disk Formatting • Low-level • Logical • Boot Block • Bootstrap • Bad-Block Recovery • Manually • Sector Sparing (Forwarding) • Sector Slipping

43. Disk Formatting • Low-Level Formatting (Physical Formatting) • Header, Trailer • Sector Number • ECC • Error detection • Soft error recovery • Data-Area • 512 bytes • Logical Formatting • Partition • One or more group of cylinders • Each partition is treated as a separate disk (example) • Logical Formatting • Storing of initial file system • Map of allocated and free space • An initial empty Directory • Cluster • Blocks are put together to increase efficiency • Disk I/O done via blocks/File I/O done via clusters • Raw disk • Some programs use the disk partition as a large sequential array of logic blocks bypassing the file system services

44. Boot Block • Bootstrap Program • Initial program to start a computer system • Initializes aspects of the system • CPU registers • Device controllers • Contents of the main memory • Starts the OS • Finds the OS Kernel on disk and loads it into the memory • Jumps to an initial address to begin the OS exec. • Stored in ROM • No need for initialization • No virus • Problem? Hard to update  solution: save the bootstrap loader in the ROM, full bootstrap on boot blocks (fixed location on HDD)

45. Booting from a Disk in Windows 2000

46. Bad-Block Recovery • Complete Disk Failure • Replace the disk • Bad Sector Handling • Manually • IDE: format, chkdsk • Special entry into FAT • Sector Sparing (Forwarding) • SCSI • Controller maintains a bad sector list • List is initialized during the low-level formatting • Controller sets aside spare sectors to replace bad sectors logically (Example) • Problem? Invalidate optimization done by disk scheduling alg. • Solution? Spare sectors on each cylinder • Sector Slipping • Move down every sector tom empty the next sector to the bad sector • Example • Soft-error: repairable by disk controller through ECC • Hard-error: lost data  back up

47. Swap-Space Management • In modern Operating Systems, “Paging” and “Swapping” are used interchangeably • Virtual memory uses disk space as an extension of the main memory • Performance decreases. why? • Swap-space management goal • To get the best throughput for the virtual memory system

48. Swap-Space Management (Cont.) • How is it used? • Depends on memory management alg. • Swapping: Load entire process into disk • Paging: Stores pages • Amount of swap space needed depends on • Amount of physical memory • Amount of virtual memory • Way virtual memory is used • Ranges from few MB to GB • Better to overestimate why? No process is aborted • Solaris: swap space = amount by which VM exceeds pageable physical memory • Linux: swap space = double the amount of physical memory • Multiple swap spaces

49. Swap-Space Management (Cont.) • Where is it located on disk? • In the normal file system • Large file within the file system • File-system routines can be used • Easy to implement but inefficient • Takes time to traverse the directory structure • Separate disk partition • Raw partition • Swap space storage manager • Uses alg. Optimized for speed rather than storage efficiency why? • Trade-off between speed and fragmentation  acceptable (data life is short) • Fixed amount of space is set aside during partitioning • Adding more space requires re-partitioning • Linux: Supports both • Who decides?

50. Swap-Space Management (Cont.) • How is it managed? • Unix • Traditional: copy the entire processes • Newer: combination of swapping & paging • Solaris1 • File-system: text-segment pages containing code • Swap-space: pages of anonymous memory such as stack or heap • Modern versions only allocate the swap space if page is forced out of the main memory