Secondary Storage CSCI 444/544 Operating Systems Fall 2008

Agenda Overview of secondary storage (disks) Disk structure Disk performance Disk scheduling Disk management RAID (Redundant Arrays of Inexpensive Disks)

Secondary Storage Secondary storage typically: is anything that is outside of “primary memory” does not permit direct execution of instructions or data retrieval via machine load/store instructions Characteristics: it’s large: GB it’s cheap: $0.45/GB it’s persistent: data survives power loss it’s slow: milliseconds to access

Disk capacity, doubled every 3+ years 25% improvement each year factor of 10 every decade Still exponential, but far less rapid than processor performance Disk capacity since 1990 doubling every 12 months 100% improvement each year factor of 1000 every decade 10x as fast as the increase of processor performance! Disk trends

Memory Hierarchy Each level acts as a cache of lower levels CPU registers L1 cache L2 cache Primary Memory Secondary Storage Tertiary Storage <1KB 64KB 4MB 2GB 1000GB TB 10 ms 1s-1hr <1ns 1 ns 4 ns 10 ns

Disks and the OS Disks are messy, messy devices errors, bad blocks, missed seeks, etc. Job of OS is to hide this mess from higher-level software low-level device drivers (initiate a disk read, etc.) higher-level abstractions (files, databases, etc.) OS may provide different levels of disk access to different clients physical disk block (surface, cylinder, sector) disk logical block (disk block #) file logical (filename, block or record or byte #)

Physical Disk Structure

Disk Controller Responsible for interface between OS and disk drive Common interfaces: ATA/IDE vs. SCSI –ATA/IDE used for personal storage –SCSI for enterprise-class storage Basic operations Read block Write block OS does not know of internal complexity of disk Disk exports array of Logical Block Numbers (LBNs) Disks map internal sectors to LBNs

Disk Operations Disk performance depends on a number of operations seek: moving the disk arm (head) to the correct cylinder –depends on how fast disk arm can move rotation (latency): waiting for the sector to rotate under head –depends on rotation rate of disk transfer: sequentially moving data from surface into disk controller, and from there sending it back to host –depends on density of bytes on disk When the OS uses the disk, it tries to minimize the cost of all of these operations particularly seeks and rotation

Disk Performance Positioning (head): Seek + Rotation Positioning time: Seek time + Rotational Delay How long to read or write n sectors? Positioning time + Transfer time (n) Implicit contract: Large sequential accesses to contiguous LBNs achieve much better performance than small transfers or random accesses

Disk Scheduling Goal: Minimize positioning time FCFS: Schedule requests in order received Advantage: Fair Disadvantage: High seek cost and rotation Shortest seek time first (SSTF): Handle nearest cylinder next Advantage: Reduces arm movement (seek time) Disadvantage: Unfair, can starve some requests

FCFS

SSTF

Disk Scheduling (II) SCAN (elevator algorithm) –move arm from one end toward the other end –service requests until reach the other end, then reverse –skews wait times non-uniformly C-SCAN (Circular-Scan) –Like scan, but only go in one direction, then start over again (typewriter) –uniform wait times LOOK and C-LOOK –similar to SCAN and C-SCAN, except stop at the last request –look for a request before continue to move in a give direction

SCAN

C-SCAN

C-LOOK

Disk Management Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write. To use a disk to hold files, the operating system still needs to record its own data structures on the disk. Partition the disk into one or more groups of cylinders. Logical formatting or “making a file system”. Boot block initializes system. Bootstrap loader program in ROM. the full bootstrap program is stored in the “boot block” at the fixed location on the disk.

Reliability Disks fail more often.... When continuously powered-on With heavy workloads Under high temperatures How do disks fail? Whole disk can stop working (e.g., motor dies) Transient problem (cable disconnected) Individual sectors can fail (e.g., head crash or scratch) –Data can be corrupted or block not readable/writable Disks can internally fix some sector problems ECC (error correction code): Detect/correct bit flips Retry sector reads and writes: Try different offset and timing combinations for heads Remap sectors: Do not use bad sectors in future

RAID RAID: multiple disk drives provide reliability via redundancy Performance: parallel access Capacity: store more data Disk striping uses a group of disks as one storage unit. RAID schemes improve performance and improve the reliability of the storage system by storing redundant data. Mirroring or shadowing keeps duplicate of each disk. Block interleaved parity uses much less redundancy. RAID turns multiple disks into one bigger, faster, more reliable disk