Operating Systems: Lecture 20 - Disk Structure, Addressing, and Scheduling, Study notes of Computer Science

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Operating Systems CMPSC 473 Storage April 3, 2008 - Lecture 20 1 Outline • Disk structure: physical and logical • Disk addressing • Disk scheduling • Management 2 Past & Present in Storage 1956: IBM 305 RAMAC - 5 MB capacity (50 disks, each 24” in diameter) 2008: Seagate Savvio 15K - 73.4 GB capacity, 2.5” diameter - can read/write complete works of Shakespeare 15 times per second 5 Storage Hierarchy L1 cache L2 cache main memory secondary storage registers expensive and fast cheap and slow tertiary storage 6 Secondary Storage • Generally, magnetic disks provide the bulk of secondary storage in systems – future alternative: solid-state drives? • e.g. MacBook Air – MEMS and NEMS(nanotech) – holographic storage • data read from intersecting laser beams www.inphase-technologies.com 7 Disk Operation • Platters start moving from rest (spinup time) – lots of mass to start moving • Heads find the right track (seek time) – arm powered by actuator motor, accelerates and coasts, slows down and settles on correct track (servo-guided) • Disk rotates until correct sector found (rotational latency) – contingent on platter diameter and RPM (Savvio 15K rotates 300 times/second) • Have to stop the platters (spindown time) 10 Addressing Disks • Old days: CHS (cylinder-head-sector) – supply physical characteristics of the disk to the operating system – it specifies exactly where on the physical disk to read and write data • Nowadays: cylinders not uniform – can store more data on outer tracks than inner tracks (zoned bit recording) • why? – function of constant angular velocity (CAV) vs constant linear velocity (CLV) found in CD-ROM 11 Logical Block Addressing (LBA) • OS sees drive as an array of blocks – first block LBA = 0, next block LBA = 1 etc. • disk firmware takes care of managing the physical location of data • Block: smallest unit of data accessible through the OS – can be the size of a sector (512 bytes) up to the size of a page ( often 4 KB): defined by kernel 12 First-come First-served (FCFS) • Service the requests in order of arrival • Head movement of 640 cylinders 15 Shortest Seek Time First (SSTF) • Min. seek time from head position (like SJF) • Head movement of 236 cylinders 16 SCAN (Elevator) Algorithm • Arm moves from one end of disk to the other then reverses (like an elevator) • Head movement of 208 cylinders 17 Choosing a Disk Scheduling Algorithm • SSTF: increased performance over FCFS • SCAN, C-SCAN: good for heavy loads – less chance of starvation • C-LOOK: good overall • File allocation plays a role – contiguous allocation limits head movement • Note: only considering seek time – rotational latency also important but hard for OS to know (doesn’t have physical drive characteristics) – drive controllers implement some queueing and request coalescing 20 Why not have drive controller do all the scheduling? • Would be more efficient, but... • OS knows about constraints that the disk doesn’t – demand paging > application I/O – write > read if cache is almost full – guaranteeing write ordering (e.g. journaling, data flushing) 21 Aside: Linux I/O Schedulers • Linus Elevator (default in 2.4 kernel) – merges adjacent requests and sorts request queue – can lead to starvation in some cases though: big push to change for 2.6 kernel • Deadline I/O Scheduler – merges & sorts request + expiration timer – multiple queues to minimize seeks while ensuring request don’t starve • Anticipatory I/O Scheduler – waits a few ms after a read request to see if another one is made (high probability); acts like deadline scheduler otherwise – loses time if wrong but big win if right 22 22 Low-Level (Physical) Formatting • divide disk into sectors for disk controller to read and write – sector numbers, error-correcting codes (ECC), other identifying information (e.g., servo control data) written to each sector • usually only done at factory – can restore factory configuration (reinitialize) 25 25 High-Level (Logical) Formatting • Before formatting, OS needs to partition the disk into 1 or more cylinder groups – why more than 1? root vs swap partitions, dual boot, etc. • write a file system onto the disk – structures such as file allocation table (FAT - DOS) or inodes (UNIX) • write the boot block (boot sector) 26 26 Boot Process • Bootstrapping starts from a process in ROM • Boot loader reads a bootstrap program from the bootblock – on PCs: Master boot record (MBR): first sector on disk (446 bytes, then 64 byte partition table) • Second-stage boot loader: program whose location is pointed to from MBR – NTLDR on Windows, LILO/GRUB on Linux • choose the partition to boot from to start to OS 27 27 Linux Swap Structures H—————————- Swap area page — slot H swap partition or swap file swap map 30 Attaching Disks to Networks • NAS: network attached storage - RPCs between host and storage – e.g., NFS (what we use), iSCSI • SAN: storage area network – multiple connected storage arrays, servers connect directly to SAN • Becoming more like each other – e.g., Open Storage Networking proposal (from NetApp) combines elements of each 31 31 SCSI vs IDE/ATA • Originally speed but with serial ATA (SATA) interface speeds have caught up • SCSI supports more drives on a bus but SATA can be beneficial for small numbers • Why pay more for SCSI? Disks manufactured differently – assumed to be server (enterprise) vs personal • often faster (e.g., 15K disks usually only SCSI) • SCSI drives better constructed (O-ring sealing, air flow, more rigidity); stronger actuator motors; more reliable • ATA cheap though: 1 TB SATA < 73 GB SCSI 32 32