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md/raid_penalties.md

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+# RAID Penalties
+
+## Contents
+
+  - [Write Penalty](#write-penalty)
+  - [Penalty Calculation](#penalty-calculation)
+  - [References](#references)
+
+
+## Write Penalty
+
+The penalty is for how we deal with the stripe (parity, etc.) where 1 means no penalty.
+
+| **RAID Level** | **Write Penalty** |
+| -------------- | ----------------- |
+| 0              | 1                 |
+| 1              | 2                 |
+| 5              | 4                 |
+| 10             | 2                 |
+
+
+## Penalty Calculation
+
+  - **raw IOPS** = disk speed IOPS \* number of disks
+  - **functional IOPS** = (raw IOPS \* write% / write penalty) + (raw IOPS \* read%)
+
+Given 5x 15k SAS drives @ 900 IOPS (180/disk) as an average, a sample calculation for RAID-5, with varying levels of write vs read percentage:
+
+```
+16% writes: (900*.16/4)+(900*.84) = 792 / 900 = 88.0% efficiency
+10% writes: (900*.10/4)+(900*.90) = 832 / 900 = 92.4% efficiency
+ 5% writes: (900*.05/4)+(900*.95) = 866 / 900 = 96.2% efficiency
+```
+
+Reducing the write percentage from 16% to 10% would yield an efficiency gain of 4.4%, from 16% to 5% yields an 8.2% gain. Writing to RAID5 and calculating parity is very costly - we haven't even talked about linear writes vs. scattered block writes. As an exercise, if the same 5-disk RAID-5 was converted to an 8 disk RAID-10 (to achieve the same capacity for data):
+
+```
+8*180 = 1440 raw IOPs
+(1440*.16/2)+(1440*.84) = 1324 / 792 = 167% efficiency
+```
+
+This is all very basic math that doesn't take into account real world load and deals with theoretical maximums based on published standards. Use a tool such as `fio` to obtain real world performance.
+
+
+## References
+
+  - <http://git.kernel.dk/?p=fio.git;a=summary>