File management and Performance

File Systems Architecture

device drivers

physical I/O (PIOCS)

logical I/O (LIOCS)

access methods

File organization and accesssequentialindex-sequentialdirect

File sharing and blocking

File sharing– access rights: read, write, execute, by user

classes.– File/record locking: simultaneous access

Record blocking– Logical vs physical record– unblocked vs blocked record– fixed-lenght vs variable-lenght

IRG

block data

IBG

block data block data

IBG IBG IRG

File allocation File allocation table (FAT) Pre allocation vs dynamic allocation Portion size

– contiguity increases performance– fixed-size portions simplify reallocation– variable-size or small fixed-size minimizes

waste of unused space for overallocation– two main alternatives:

variable, large contiguous portions (performance)

small fixed portions (flexibility) File fragmentation and reorganization

Redundant Array of Independent/ Inexpensive Disks (RAID)

• RAID is a set of physical disk drives viewed by the OS as a single logical drive

• Data are distributed across the physical drives of an array

• Redundant disk capacity is used to store parity information, which guarantees data recovery

• RAID Levels: various levels of performance and redundancy

– the disks are divided in strips: physical blocks, sectors, or similar

– the strips are mapped round robin on the array– I/O requests can be processed in parallel

strip 0strip 4strip 8strip 12

strip 1strip 5strip 9strip 13

strip 2strip 6

strip 10strip 14

strip 3strip 7

strip 11strip 15

Performance How effective is the OS in managing resources?

Overall performance measures:– turnaround time– response time– throughput– workload– capacity– utilization

Bottlenecks

Evaluation Techniques

Timings (hardware) Instruction mix (software mix) Kernel (typical program) Benchmarks (comparison between systems) Synthetic programs Analytical models and simulation Commercial performance packages

Performance enhancement coprocessing parallel processing the RISC vs CISC controversy

– CISC frequently used code as (complex) instructions increase lower level programming productivity extensive market penetration

– RISC very few of the complex instructions are used (empirical

studies) therefore a reduced instruction set is more efficient programs are 20% larger , requiring more disk space and

memory much more registers are needed, but speed of programs

written for RISC is close to twice of equivalent for CISC. facilitate scalability