Effective memory access time:
EA = HC + (1-H)S
Where H = Hit ratio
Hit ratio = references satisfied by cache / total references
C = cache access time; S = memory access time
Effective disk buffer cache access time:
EA = HC + (1-H)S
Where H=Hit ratio C = cache access time; S = memory access time
Disk access time = seek time (tracks)+ rotational time (section) + transferlength / bandwidth
Storage System
-
MTBF (Mean Time Between Failure):
Time between failures.svg
Describes the expected time between two failures for a repairable system
- Time between failures = {down time - up time}
- C12pture.PNG
-
Mean Time to Repair
(Total down time) / (number of breakdowns) -
Mean Time to Fail:
For the product that cannot be repaired.
RAID (Redundant Array of Independent)
-
RAID 0
-
Block Level Striping(4k or 8k bytes of storage)
-
Provides balanced I/O of disk, throughput approximately doubles
-
Any one of the disk failure could cause the MTTF reduces by a factor of 2
-
RAID 1
-
Block Level Mirroring
-
Provides higher read throughput but lower write throughput
-
MTTF increases substantially (MTTF^2)
-
RAID 2
-
Bit Level Mirroring(4k or 8k bytes of storage)
-
Provides higher transfer rate
-
MTTF increases substantially (MTTF^2)
-
RAID 3
-
Byte Level Striping (Parity)
-
Provides higher transfer rate as RAID 0
-
MTTF increases substantially (1/3 of RAID 1 = MTTF^2/3)
-
P0 is parity (exclusive-or) for b0 and b1. Knowing any two of the disk data could recover the third data.
-
RAID 4
-
Block Level Striping (Parity)
-
Similar to RAID 3
-
Disk 3 has a lot more stress because it has more writes than either disk 1 and disk 2 as the parity has to be updated on every write
-
RAID 5
-
Block Level Striping (Parity)
-
Single disk failure tolerant array
-
No dedicated disk for parity blocks, parity blocks are also striped
-
MTTF is same as RAID 3
Capture.PNG -
RAID 6
-
At least 5 disks including two parity blocks used
-
Reliability is of the order of MTTF^3/10
-
p0 and p1 are the parity blocks for blocks A0,A1,A2
网友评论