Maximizing Disk Performance: Features, Scheduling, and RAID Explained

1. CS 346 – Chapter 10 • Mass storage • Advantages? • Disk features • Disk scheduling • Disk formatting • Managing swap space • RAID

2. Disks • Anatomy (Figure 10.1) • Sector, track, cylinder, platter • Read/write head attached to arm, attached to arm assembly • Head quickly reads binary data as: orientation of iron ions or reflectiveness of surface • Example: CD • About 25,000 tracks, 50 sectors per track  1 bit occupies about 1 square m • Entire CD can be read in about 7 minutes on a 12x speed drive • But usually we don’t read entire disks 2 aspects dominate access time: • Seek time: proportional to square root of seek distance • Rotational latency

3. Some specs

4. Disk scheduling • Common problem is a backup of disk requests • Disk queue • When disk is ready, in what order should it do the disk requests? Similar to problem of scheduling CPU • Pending jobs are classified by which track/cylinder they want to access • Ex. 4, 7, 16, 2, 9, 1, 9, 5, 6 • Several disk scheduling algorithms exist • Simple approach: first-come, first-served • Total head movement = ? • Want to reduce total seeking time or head movement: avoid “wild swings”. • Would be nice not to finish at extreme sector number.

5. Scheduling (2) • Shortest seek first • For 4, 7, 16, 2, 9, 1, 9, 5, 6: After serving track 4, where do we go next? Total head movement = ? • Very good but not optimal • Elevator algorithm (“scan” method) • Pick a direction and go all the way to end, then come back and handle all other requests. • Better than Shortest Seek in our example? • Circular scan • Same as elevator algorithm BUT: when you reach the end you immediately go to the other end without stopping for requests. In other words, you only do work as head is moving in 1 direction. • Look scheduling: modify elevator & circular scan so you only go as far as highest/lowest request

6. Disk mgmt • Low-level (physical) formatting • Dividing disk medium into sectors • Besides data, sector contains error-correcting code • Later, disk controller will manipulate individual sectors • High-level (logical) formatting • Record a data structure for file system on disk • Partition groups of cylinders if desired • Associate adjacent blocks into logical clusters to support file I/O • “Sector sparing”: compensate for bad blocks! • Maintain list of bad blocks; replace each with a spare one • Boot from disk: boot blocks in predefined locations contain system code to load  “boot partition” of drive

7. Swap space • Recall: used in virtual memory to store pages evicted from RAM • Faster to return to RAM than loading from file from scratch • In effect: disk space is now being used as extension of main memory, the very essence of VM • Logically a separate partition of the disk from the file system • When process started, it’s given some swap space • Swap map: kernel data structure to track usage • Associate an counter value with each page in swap area • 0 means that page is available to swap into • Positive number: number of processes using that swapped-out data (> 1 means it’s shared data)

8. RAID • Increasingly practical to have several disks on a system • But increases probability & mean time to failure • RAID = “redundant array of independent disks” • Redundancy: fault tolerance technique • Six “levels” or strategies of RAID: use various combinations of fault tolerant techniques • Typical RAID techniques in use • Striping a group of disks: split bits of each byte across disks Or block-level striping: split blocks of a file… • Mirroring another disk • Store parity (error-correcting) bits on another disk • Leaving some disks empty until needed to replace failed disk

9. RAID levels Various combinations of techniques… For example: • RAID 0 – block striping; no mirroring or parity bits • RAID 1 – add mirrored disks • RAID 2, 3, 4 – extra disks store parity bits • If 1 disk fails, remaining bits of each byte and error-correction bit can be used to construct lot bit of each byte. • RAID 3 – bit-interleaved parity • RAID 4 – block-interleaved parity • RAID 0+1 – a set of disks is striped, and then the stripe is mirrored to another disk • RAID 1+0 – disks are mirrored into a pair of disks. This pair is then striped.

10. RAID extensions • RAID is designed just to detect & handle disk failure • Does not prevent/detect data corruption, etc. • Could be pointing to wrong file, wrong block • Checksum for data and metadata on disk • Ex. For each disk block, how many bits are set? • Store with pointer to object (See Figure 10.13) • Detect whether it has changed. Grab correct data from the mirror. • RAID also somewhat inflexible because its techniques require a certain number of disks. What to do?