Disks and Storage Systems

Disks and Storage Systems CSE451 Andrew Whitaker

Big Picture • OS provides abstractions to allow physical HW resources to be shared • CPU sharing with threads • Memory sharing with virtual memory • Disk sharing with files

Physical disk structure • Disk components • platters • surfaces • tracks • sectors • cylinders • arm • heads sector track surface cylinder platter arm head

Disk Interface (as seen from the OS) • Disk exposes a contiguous array of logical block numbers • Internally, the disk maps these to cylinder, head, sector

Disk Size and Cost • Cost per gigabyte has plummeted • Now approximately $ .33 per Gigabyte

Disk trends • Disk capacity, 1975-1989 • doubled every 3+ years • 25% improvement each year • factor of 10 every decade • 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 an improvement as processor performance!

Components of Disk Access Latency • Seek time: moving the disk arm to the correct cylinder • Rotation time: waiting for the sector to rotate under head • Depends on rotation rate of disk • Transfer time: transferring data from surface into disk controller, and from there into main memory

Implications of Disk Performance • Large transfers are faster (per byte) than small transfers • Contiguous transfers are faster than scattered transfers • Contiguous transfer rate is 500X random access rate

Disk Partitions • Each physical disk can be split into logical disks, called partitions Partition 1 Partition 2 Partition 3 • Partition table provides sizes, types • Located in the first disk sector • Master Boot Record

Disks and File Systems • Each partition can contain a file system • Examples: NTFS, ext3, FFS • UNIX mount command shows file system  partition mapping /dev/sda2 on / type ext3 (rw) /dev/sda5 on /usr type ext3 (rw,nodev) /dev/sda6 on /var type ext3 (rw,nosuid,nodev) /dev/sda7 on /tmp type ext3 (rw,nosuid,nodev) proc on /proc type proc (rw)

Accessing the Disk • Disk access is provided by a block device driver • Provides a function to enqueue a request Privileged User Apps File Systems Enqueue request Device Driver Read/write sectors Disk

What Happens on Enqueue Request? • OS maps from <partition, offset> to a logical block number • Make sure the offset is in bounds! • The request is added to a pending request queue • Common optimization: clustering • If the request is adjacent (on disk) to another request, merge them • Request scheduling • OS chooses an order for pending requests (see next slides) • Once the device is free, the device driver dispatches one or more pending requests

Disk Scheduling • Problem: Given a set of pending disk requests, choose an ordering • Goals (Quite possibly conflicting) • Maximize throughput • Minimize waiting • Provide fairness • Avoid starvation

Policy #1: First-come, First-serve • Disk requests are dispatched in the order they arrive • Performance of FCFS is highly dependent on the workload • Performs well if disk requests arrive in contiguous order • Performs poorly if requests arrivals are scattered across disk

Policy #2: Shortest-seek-time-first • Schedule the request that is closest to the current head position • Advantage: much better performance than FCFS • Disadvantage: bias, starvation • Requests in the middle cylinders get better service • Requests in outer cylinders can starve

Policy #3: SCAN Scheduling • Head continuously moves back and forth across the disk • Sometimes called “elevator algorithm” • Advantages: starvation-free, reasonable performance, easy to implement (sort)

Looking Ahead: File Systems • One proposal: use disk partitions for files • Use one disk partition per file • Is this a good idea? Why or why not? foo.txt bar.txt zag.txt

Disks and Storage Systems