Week 12 - Disk Performance Flashcards
The average time to read or write a 512B sector for a disk. Rotating at 15,000 RPM with average seek time of 4ms, a 100MB/sec transfer rate, and a 0.2 ms controller overhead.
What is the disk access time?
Avg. disk read/write = TS+TR+TT+TC
Ts=4ms
Tr = 1/2r = 2ms
Tt = 512B / 100MB / sec = 0.005ms
Tc = 0.2 ms
T total = Ts + Tr + Tt + Tc = 4+2+0.005+0.2=6.2ms
If the measured avg seek time is 25% of the advertised avg seek time then
Avg disk read/write = 1.0 + 2.0 + 0.005 + 0.2 = 3.2 ms
What hazards are there when it comes to pipelining?
Different instructions need the same resources (structural hazards)
- Different instructions need results from other instructions (data
hazards)
- Different instructions execute depending on other instructions
(branch instructions, control hazards)
The instruction pipeline has the following stages: instruction fetch (IF), instruction decode (ID), operand fetch (OF), perform
operation (PO) and writeback (WB). The IF, ID, OF and WB stages take 1 clock cycle each for every instruction. Consider a sequence of 100 instructions. In the PO stage, 40 instructions take 3 clock cycles each, 35 instructions take 2 clock cycles each, and the remaining 25 instructions take 1 clock cycle each. Assume that there are no data hazards and no control hazards.
The number of clock cycles required for completion of execution of the sequence of instruction is ______.
1 instr: 1 + 1 + 1 + 3 + 1 = 7 clock cycles
39 instr: 39 x 3 cycle = 117
35 instr → 35 x 2 cycle = 70
25 instr → 25 x 1 cycle = 25
Total 219 cycles
What is RAID?
Redundant Array of Inexpensive Disks.
Arrays of small and inexpensive disks
- Increase potential throughput by having many disk drives:
- Data is spread over multiple disks
- Multiple accesses are made to several disks at a time
What is parity?
Parity: a technique that checks whether data has been lost or written over when it
is moved from one place in storage to another (or when transmitter between
computers);
What is performance bottleneck?
: overloaded network or the state of a device in which
one component is not able to keep pace with the rest of the system, thus slowing
overall performance
What is reliability in terms of disk performance?
probablity that the system will produce correct outputs up to some given time period T.
What is redundancy in terms of disk performance?
duplication of critical components or functions of a system with the intention of increasing reliability of the system;
What is data striping?
technique of segmenting logically sequential data, such as a file, so that consecutive segments are stored on different physical storage devices;
What is Round Robin?
one of the algorithms employed by a process and network schedulers in computing (time slices assigned to each process in equal portions and in circular order, handling all processes without priority – cyclic executive);
Methods of RAID 0, 1, 2, 3, 4, 5, 6?
RAID 0 | data striping
RAID 1 | mirroring
RAID 2 | bit level striping + Hamming Code
RAID 3 | byte level striping + parity scheme
RAID 4 | block level striping + dedicated parity disk
RAID 5 | block level striping + distributed parity
RAID 6 | distributed double parity
Consider a 4-drive, 200GB-per-drive RAID array. What is the available data storage capacity for each of the RAID levels 0, 1, 3, 4, 5, and 6?
RAID 0: Has full capacity, thus the available data storage capacity is 4 · 200𝐺𝐵 = 800GB.
RAID 1: Uses N/2 drives for data storage, which means that
the available data storage capacity is 4/2 x 200 GB = 400 GB
RAID 3, 4, 5: The available data storage capacity is (N-1)x200GB = 600 GB
RAID 6: The available data storage capacity is (N-2)x200 = 400 GB
