Midrange Performance Fundamentals

Question 1

What is Cache Dirty Size?

Answer 1

capacity of mirrored cache holding data yet to be committed to drive

Question 2

What is a Vault?

Answer 2

internal area within the storage system that is required for write cache dumping

Question 3

What does it mean to coalesce IO?

Answer 3

to combine multiple smaller IO into a larger IO

Question 4

What is locality?

Answer 4

active data regions relative to logical block address range or aggregate of ranges

Question 5

What parts of RAID affect performance the most?

Answer 5

data copies incur additional drive IO (RAID 1/0)
parity generation will take CPU utilization (RAID 5 and 6)
parity operations will take drive utilization (RAID 5 and 6)

Question 6

Why are SSDs capable of much higher IOPS?

Answer 6

multiple internal channels allow multiple operations in parrallel

Question 7

What can affect the performance on an SSD?

Answer 7

wear leveling
garbage collection
endurance levels

Question 8

What is an example of availability?

Answer 8

10 drives have higher performance potential than 5 drives but a lower availability due to the higher probability of multiple drive failures

Question 9

What are the ways to choose a RAID type?

Answer 9

fast - for a given number of spindles how good is the relative performance

cheap - how much usable capacity is there - compared to the raw capacity

available - how unlikely is it that the next drive failure will cause data loss

Question 10

What is the write penalty for each RAID group?

Answer 10

RAID 5 - 4 - old/new data, old/new parity 1
RAID 6 - 6 - old/new data, old/new parity 1, old/new parity 2
RAID 1/0 - 2 - old/new data

Question 11

What causes more drive load on parity RAID types?

Answer 11

random writes

Question 12

What is almost always true about midrange storage and performance?

Answer 12

random reads almost always a cache miss

Question 13

What does a smaller IO help with?

Answer 13

transactional capability and faster service times

Question 14

What is the benefit of larger IO?

Answer 14

affords higher bandwidth - but large random can result in additional drive IO if spanning multiple stripe elements

Question 15

What does random IO benefit from?

Answer 15

lots of drives

Question 16

What is the relationship between concurrency and storage consolidation?

Answer 16

traditionally leads to more concurrency (multiple hosts pointing to one storage system)

Question 17

Why does server virtualization typically reduce concurrency?

Answer 17

consolidating servers means consolidating paths to the storage

Question 18

What benefit does read caching have on performance?

Answer 18

previously read data is stored in cache - LRU data is discarded when cache pages are needed

much shorter response time to service a cached read

Question 19

What benefit does write caching have on performance?

Answer 19

hitting write cache typically faster than going to HDD (as long as cache isn’t saturated)

for a small IO under 1ms vs 5-10ms going to disk

insulates host IO from drive response (RAID overhead, drive type, etc.)

can coalesce chunks of data up to entire stripes

Question 20

What may not benefit from write cache?

Answer 20

a storage system that has a lot of non-parity writes and uses SSDs

Question 21

What benefit does IO ordering/processing have on performance?

Answer 21

while data is stored in cache controller can optimize how it will be sent to the drives

order blocks to minimize drive seeking during reads and writes

coalesces multiple contiguous blocks into a single large IO