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Lecture 22: Mass Storage Lecture 22: Mass Storage Fall 2019 Jason Tang Slides based upon Operating System Concept slides, http://codex.cs.yale.edu/avi/os-book/OS9/slide-dir/index.html Copyright Silberschatz, Galvin, and Gagne, 2013 "1 Topics • Mass Storage Systems! • Disk Scheduling! • Disk Management! • Boot Process "2 Mass Storage Systems • After startup, OS loads programs from secondary storage into main memory:! • ROM / EPROM / EEPROM / FPGA! • Magnetic hard disk! • Non-volatile random-access memory (NVRAM) ! • Tape drive ! • Or others: boot CD, Zip drive, punch card, … "3 EPROM • Erasable programmable read-only memory! • Manufacturer or OEM burns image into EPROM! • Use in older computing systems and in modern embedded systems "4 Magnetic Hard Disk (HDD) • Spindle motor spins a stack$ of platters coated with$ magnetic material! • Spins from 5400 to over$ 10000 RPMs! • Actuator motor moves a disk$ head over the platters, to$ sense polarity of the track$ underneath https://www.technobu%alo.com/2012/11/24/western-digital- "5 expands-high-performance-wd-black-hard-drive-line-to-4tb/ Magnetic Hard Disk (HDD) • Transfer rate: rate which data flow between drive and computer! • Positioning time (random-access time): time to move disk arm to desired cylinder (seek time) plus time for desired sector to rotate under disk head (rotational latency)! • Head crash: when disk head hits platter! • Attached to computer via a bus: SCSI, IDE, SATA, Fibre Channel, USB, Thunderbolt, others! • Host controller in computer uses bus to talk to disk controller "6 Magnetic Hard Disk Performance • Average access time = average seek time + average latency! • Server-grade hard disks average 5 ms (3 ms seek time + 2 ms latency)! • Average I/O time = average access time + (transfer size / transfer rate) + controller overhead! • Example: transfer 4 KB data with 9 ms average access time, 1 Gb/s transfer rate, 0.1 ms controller overhead! = 9 ms + (4 KB & (1 GB / 1000&1000 KB) & (8 b / 1B) / (1 Gb/s)) + 0.1 ms! = 9.100032 ms (or about 100,000 times slower than modern RAM) "7 Non-Volatile RAM (NVRAM) • Used in modern computers for secondary storage, like a magnetic hard drive! • Also known as flash memory, flash drive, or solid-state drive (SSD)! • Two variants: NAND flash (most common) or NOR flash! • Requires less power, much faster than magnetic hard disk! • Does not su%er from head crash! • Block erasure: must erase entire block at a time! • Memory wear: may only erase the same block finite times (usually over 10,000) "8 Tape Drive • Early read/write secondary storage medium! • Linear search: tape drive had to fast-forward or rewind spool of tape to correct place; very slow! • Can hold up to 200 TB! • Transfer rate on order of 140 MB/s$ (40 times slower than hard disk)! • Origin of tar (“tape archive”) command "9 Disk Structure • Addressed as large 1-dimensional array of logical blocks! • Block is smallest unit of transfer; HDD is usually 512 or 4096 bytes, NAND flash anywhere from 512 bytes to 128 KiB! • On HDD, sector 0 is first sector on first track on outermost cylinder! • Logical to physical addressing tricky, due to bad sectors! • For HDD, non-constant number of sectors per track due to constant platter rotational speed "10 I/O Scheduling • Just as OS has a process scheduler to decide which process to run next, OS has an I/O scheduler to decide which disk operation to perform next! • On HDD, minimize seek time, by decreasing physical distance that platter must rotate and for disk arm to move to correct cylinder! • On SSD, combine requests a%ecting the same block! • Disk bandwidth: total number of bytes transferred, divided by total time between start of first request to completion of last transfer! • While data being transferred via DMA, OS can do other things "11 Disk Scheduling • Disk I/O request includes input or output mode, disk address, memory address, number of sectors to transfer! • OS maintains queue of requests! • Idle disk can immediately work on I/O request, while requests are queued for a busy disk! • Optimization algorithms only make sense when a queue exists! • HDD controllers have small bu%ers and can manage a queue of I/O requests "12 FCFS Scheduling (HDD) • Example: requests for cylinders 98, 183, 37, 122, 14, 124, 65, 67; head is currently on cylinder 53! • Requests serviced first-come, first-serve! • Note wild swing between cylinders 37 to 122 to 14; would be faster if 37 and 14 were serviced consecutively "13 SSTF (HDD) • Shortest Seek Time first selects request with minimum seek time from current head position! • Form of shortest-job first scheduling, but may starve a request "14 SCAN (HDD) • Disk arm starts at one end of disk, moves towards other end, servicing requests until it reaches other end; head then reverses direction! • Also known as elevator algorithm! • Works well if requests are uniformly dense; a large density of requests at other end of disk will wait the longest "15 Circular SCAN (C-SCAN) (HDD) • More uniform wait time than SCAN! • When head reaches one end, return to beginning of disk instead of reversing direction! • Treats cylinders as circular list that wraps around from last cylinder to first "16 LOOK and Circular-LOOK (C-LOOK) (HDD) • Arm only goes as far as last request in each direction, then reverses direction immediately, without going all of the way to end of disk! • C-LOOK is LOOK with circular list "17 Disk Management (HDD) • Low-level formatting: dividing a disk into sectors that disk controller can read and write! • Each sector holds header information, data, plus error correction code (ECC)! • Usually done by manufacturer! • Usually, disk is partitioned into one or more groups of cylinders, where each partition treated as a logical disk! • Logical formatting: creating a file system "18 Disk Management (SSD) • On flash drives, can flip a single bit from 1 to 0, but reverse not physically possible! • Must instead erase entire flash sector (flip all bits in that sector from 0 to 1)! • Erasing is slow, from 1 ms per sector for NAND flash up to 5 s for NOR flash! • No such thing as low-level formatting a flash drive, though file systems still exist! • File systems optimized for SSDs operate very differently than ones designed for HDDs "19 Bad Blocks • For HDD, bad blocks discovered during low-level initialization! • HDD controller automatically skips over that block when formatting disk! • For SSD, bad blocks discovered during erase, when a bit stays stuck at 0! • OS or SSD controller marks block as bad, so that it will not be used later! • Bad blocks can also be found during operations, when controller/OS calculates a di%erent checksum during a read! • Spare sectors: extra space not normally allocated, used when replacing a bad sector "20 BIOS versus UEFI • At startup, ROM firmware loads OS from a known location within secondary storage into main memory Feature BIOS UEFI Floppy, hard disk, Any, including Storage System CD-ROM PXE boot Partition Table MBR GPT Maximum Hard 2.1 TB 9.4 ZB Disk Size Security None Secure Boot "21 BIOS versus UEFI https://phoenixts.com/blog/uefi-vs-legacy-bios/ "22 BIOS versus UEFI http://teck-in.blogspot.com/2013/09/who-invented-uefi.html "23.
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