PowerMax and VMAX All Flash Solutions Design (Fibre Channel Considerations)

Question 1

What is required for FCP data path?

Answer 1

two round trips for each write

Question 2

How do writes work w/ FCP?

Answer 2

initatior issues write command
receiver initiates transfer ready
initiator sends data
receiver sends status good

Question 3

Why is FC still fast even though it needs two round trips per write?

Answer 3

normal host to SAN operations are short distance

latency of two writes is negligible

Question 4

What is the latency of FC replication?

Answer 4

1ms latency per 200km

4ms latency per write for sites 200km away

Question 5

What is Write Acceleration/Fast Write?

Answer 5

performance enhancement feature offered by some FC products

goal is to minimize latency caused by the two round trips - increases performance for synchronous replication

Question 6

How is Write Acceleration/Fast Write initiated?

Answer 6

intelligent switches have additional buffers and switches at either end for replication have to coordinate operations

Question 7

What is the latency of Write Acceleration/Fast Write?

Answer 7

equivalent of one round trip

Question 8

What effect does Write Acceleration/Fast Write have on SRDF/S?

Answer 8

will always increase performance of SRDF/S

Question 9

What effect does Write Acceleration/Fast Write have on SRDF/AR or adaptive copy?

Answer 9

improves SRDF throughput by 20%

Question 10

What effect does Write Acceleration/Fast Write have on SRDF/A?

Answer 10

not a lot of performance increase since SRDF/A sends a continous stream of writes until the cycle is empty

if links are kept full there is no waiting for round-trip delay and SRDF/A performance not impacted by two round trips

Question 11

When will Write Acceleration/Fast Write increase SRDF/A performance?

Answer 11

when there are gaps on the links

due to retransmissions after link errors

or small writes that throttle SRDF throughput due to hitting IO per second limit on links

Question 12

What are best practices for implementing FC in PowerMax/VMAX environments?

Answer 12

use Dell EMC SiRT or 3rd party Fast Write feature

don’t use both simultaneously

connect host and storage pairs to same switch to reduce latency

Question 13

What does FC use for flow control?

Answer 13

BB_Credit

Question 14

How does BB_Credit work?

Answer 14

providing for frame acknowledge via R_RDY primitive defined in the FC1 Layer

BB_Credit management may occur between any two FC ports that are connected

Question 15

What are the two most common BB_Credit connections?

Answer 15

between an N Port and F Port

Between two E Ports

Question 16

What is required for transmitting ports to send frames?

Answer 16

only if BB_Credits are greater than 0

an R_RDY is sent from the receiving port to the transmitting port for every available buffer on the receiving side

Question 17

What is the relationship between BB_Credits and long distance replication?

Answer 17

insufficent BB_Credits will result in the degradation of throughput over long distance links

longer the link the more frames must be sent continously down the link to prevent to prevent this degradation

Question 18

How are BB_Credits sized in FC connections?

Answer 18

sized for 100% utilization on long distance links

switches at either end must support the minimum required level of BB_Credits

recommended to add 20% buffer to account for traffic spikes

Question 19

How are BB_Credits calculated on a switch?

Answer 19

on a per port basis

Question 20

What are the methods to extend FC connections?

Answer 20

long wave laser optics w/ ISLs

FC traffic over CWDM/DWDM/SONET

FCIP

Question 21

What is fabric isolation?

Answer 21

goal is to define fabrics in a way that events occuring in one fabric will not impact other fabrics

Question 22

Why use fabric isolation with FC based replication?

Answer 22

isolating traffic locally while minimizing traffic across the WAN leads to higher performing remote replication

Question 23

How can fabric isolation be achieved?

Answer 23

using dedicated switches to form the link across the WAN

Question 24

What is a typical scenario for FC remote replication fabric isolation?

Answer 24

each site will have its operation fabric where hosts access storage

a third fabric is created for traffic across the WAN - not connected to local/remote fabrics

disadvantage of this method is inability to share resources between local and remote sites

Question 25

What is a FC routing method of FC remote replication?

Answer 25

connects two physically separate fabrics to enable shared access to storage resources from any fabric

only the shared resources will be routed across the WAN

Question 26

What is FCIP?

Answer 26

tunneling protocol that allows geographically distributed FC SANs to be connected over local IP

each interconnection is called a FCIP Link

Question 27

How does FCIP work?

Answer 27

no translation of FCIDs

frames encapsulated into IP payloads

Question 28

What is the size of FC frames?

Answer 28

typically 2148 bytes w/ MTU size of 1500

can get to MTU 9000 but MTU must be set on local and remote ports the same

Question 29

What is TCP/IP used for in FCIP?

Answer 29

underlying transport to provide congestion control and in-order delivery of error-free data

Question 30

What is ISL Trunking?

Answer 30

allows multiple ISLs to function as a single ISL

aggregates bandwidth of all the ISLs

achieves dynamic load balancing between ISLs

Question 31

When should you implement ISL Trunking for FC replication?

Answer 31

when the (# of remote ports connected to a switch X port speed) + other traffic > (# of ISLs X ISL Speed)

when any ISL is observed to be over-utilized while an adjacent ISL is minimally utilized

Question 32

What is Exchange-Based Trunking?

Answer 32

available on Brocade and QLogic switches

works by distributing exchanges evenly over a group of ISLs

Question 33

What is Open Trunking?

Answer 33

works by making periodic changes to the hardware routing tables based on amount of time being spent at zero BB transmit credit and percent utilization measurement

Question 34

What is Frame Based Trunking?

Answer 34

available on B Series Brocade

involves aggregation of several ISLs between any two adjacent switches into one logical unit so FC frames can be evenly distributed across ISLs