Outline for 2/27/98

• Last time: File Workloads - How people use files.
• Administrative:
  • Read Distributed System chapters
  • Questions? Problems?
• Objective:
  • File caches (avoiding the disk)
  • Dealing with disks.

What to do about Disks?

• Disk scheduling
  • Idea is to reorder outstanding requests to minimize seeks.
• Layout on disk
  • Placement to minimize disk overhead
• Build a better disk (or substitute)
  • Example: RAID

Disk Scheduling

• Assuming there are sufficient outstanding requests in request queue
• Focus is on seek time - minimizing physical movement of head.
• Simple model of seek performance Seek Time = startup time (e.g. 3.0 ms) +
  N (number of cylinders ) *
  per-cylinder move (e.g. .04 ms/cyl)

Policies

• Generally use FCFS as baseline for comparison
• Shortest Seek First (SSTF) -closest
  • danger of starvation
• Elevator (SCAN) - sweep in one direction, turn around when no requests beyond
  • handle case of constant arrivals at same position
• C-SCAN - sweep in only one direction, return to 0
  • less variation in response

Layout on Disk

• Can address both seek and rotational latency
• Cluster related things together
  (e.g. an inode and its data, inodes in same directory (ls command), data blocks of multi-block file, files in same directory)
• Sub-block allocation to reduce fragmentation for small files
• Log-Structure File Systems

Log-Structured File Systems

• Assumption: Cache is effectively filtering out reads so we should optimize for writes
• Basic Idea: manage disk as an append-only log (subsequent writes involve minimal head movement)
• Data and meta-data (mixed) accumulated in large segments and written contiguously
• Reads work as in UNIX - once inode is found, data blocks located via index.
• Cleaning an issue - to produce contiguous free space, correcting fragmentation developing over time.

Build a Better Disk?

• "Better" has typically meant density to disk manufacturers - bigger disks are better.
• I/O Bottleneck - a speed disparity caused by processors getting faster more quickly
• One idea is to use parallelism of multiple disks
• Striping data across disks
• Reliability issues - introduce redundancy

RAID

Redundant Array of Inexpensive Disks

RAID Level 5