1 Operating Systems Part VI: Mass- Storage Structure

2 Disk Structure Platter Cylinder Track Sector

3 Data Transfer Seek time: time to get to the track/cylinder containing the sector Rotational latency: time to get to the desired sector Disk bandwidth: number of bytes transferred per unit time (a.k.a. transfer rate)

4 Disk Scheduling FCFS (First Come First Served) – Simplest to implement – Intrinsically fair – Does not provide the fastest service – Example 98,183,37,122,14,124,65,67 Assume start at cylinder 53

5 Disk Scheduling FCFS (First Come First Served)

6 Disk Scheduling SSTF (Shortest Seek Time First) – Selects the request with the minimum seek time from the current head position – Essentially a form of SJF – May cause starvation – Not optimal, but substantial improvement over FCFS

7 Disk Scheduling SSTF (Shortest Seek Time First)

8 Disk Scheduling SCAN Scheduling – Sometimes called the elevator algorithm – Starts at one end of disk and moves toward other end, servicing requests as it reaches each cylinder, until it gets to other end – Head continuously scans back and forth – Disadvantage: When head reverses direction, what happens to density of request in vicinity?

9 Disk Scheduling SCAN Scheduling

10 Disk Scheduling C-SCAN Scheduling – Variation of SCAN but returns to beginning of disk without servicing requests on return trip – Treats cylinder as circular list that wraps around from final cylinder to the first one

11 Disk Scheduling C-SCAN Scheduling

12 Disk Scheduling LOOK and C-LOOK Scheduling – Variations of SCAN and C-SCAN – Goes as far as final request in each direction then reverses course without going all the way to the end – Looks for request before continuing in a direction

13 Disk Scheduling C-LOOK Scheduling

14 Selection of Disk Scheduling Algorithm Depends heavily on number and type of request Can be heavily influenced by file allocation method (contiguous vs. linked/indexed) Location of directory (first/middle/last cylinder) -> better cached SSTF or LOOK reasonable default

15 Disk Management Formatting – Low-level formatting (physical formatting) Fills disk with special data structure (header, data area, and trailer) for each sector (usually 512 bytes) -> usually done in factory Implements error-correcting code (ECC) which is updated with value calculated from all bytes in sector When sector is read, ECC is recalculated and compare with stored value -> mismatch = corruption

16 Disk Management Formatting (continued) – Partitioning Divides disk into one or more groups of cylinders Tools: fdisk, Partition Magic, System Commander – Logical formatting Creation of file system (FAT/inode, free-space mapping, initial empty directory) Raw disk: large sequential array of disk blocks without any file system structure (used by some applications) -> bypasses directory structure, file names, space allocations, and other file services

17 Disk Management Boot Block – Boot short for bootstrap – Boot program stored in ROM Fixed location and needs no initialization Cannot be infected by virus Problem: changing boot code requires changing ROM hardware chips – Solution: ROM -> boot disk (w/ boot partition)-> O/S kernel

18 Disk Management Bad Blocks – Unavoidable due to moving parts and proximity of R/W head to platter surface – Data in bad blocks are usually lost – Handled manually in simple disks (IDE) Format scans for bad blocks and locks them away Chkdsk and scandisk do the same but used in normal operation – SCSI disks “smarter” in handling bad blocks Uses sector sparing or forwarding Sector slipping sometimes used in place of sparing

19 Swap Space Goal: provide best throughput for virtual memory system Swap space use – Varies depending on O/S (e.g. entire process image, pages pushed out of MM swapped, etc.) – Some O/Ss allow multiple swap spaces – Safer to overestimate than to underestimate size Swap space location – Part of the file system (Windows) – Separate disk partition (normally raw device, e.g. Unix)

20 RAID Structure Redundant Arrays of Inexpensive Disks Concept now extended to other devices such as tapes Redundancy solves problem of reliability – Duplicate each disk (mirroring/shadowing) Performance improvement via parallelism – Mirroring doubles read access times – Data striping: splitting data across multiple disks (bit- level, block-level, sector-level, etc.)

21 RAID Levels Mirroring: high-reliability but expensive; striping: high-speed but does not improve reliability RAID levels provide a scheme that combines mirroring and striping – Cost-performance trade-offs (hence, levels) – Example: RAID 0 (block striping but without mirroring), RAID 1 (mirroring), RAID 0 + 1

22 Disk Attachment Host-attached storage – May be IDE, SCSI, or RAID device Network-attached storage – Provides convenient way to share pool of storage and data – Consumes network bandwidth – More efficient if Storage Area Network (SAN) is implemented (using storage rather than networking protocols)

23 Tertiary Storage Built with removable media Low-cost is defining characteristic Removable disks – Magnetic (e.g. floppy, zip drives, hot- swappable hard-disks) – Optical WORM (Write Once Read Many, e.g. CD-R) Phase-change (e.g. CD-RW) – Magneto-optical

24 Tertiary Storage Tapes – Off-line: sits on shelves – Near-line: uses robotic arms -> between off- line and on-line (e.g. disks) Future technology – Holographic storage: records holographic photographs on special media – Micro-electronic mechanical systems (MEMS) -> produces small storage machines

25 Tertiary Storage Performance Speed Reliability Cost