Synchronous Storage Replication Process

[Jordi Benet]

Hi,

I have a question regarding storage replication for synchronous mode. If I describe the process in detail having twin Data Centers called DC-A and DC-B we have:

1- The server in DC-A writes to the DC-A disk.

2-  Before the server in DC-A receives an ACK from the DC-A disk, this data is replicated to the other DC disk, DC-B Disk.

3-  To replicate the data to the other DC we will first ask to the other DC before passing through the DCI “are you ready to receive”?

4-  We will get the approval from DC-B that is ready

5-  Then DC-A will send the data through the DCI to arrive to DC-B.

6-  DC-B disk receives de data and then DC-B disk must send an ACK to DC-A Disk

7-  Once the ACK is received at DC-A disk, DC-A disk will then send the ACK to the server in DC-A which originated the write action.

My question is if step 3 and 4 sometimes are counted and sometimes not. I should counted or not? This decision is important for me because it affects by a factor of 2 the distance between the twin DCs.

Thank you very much for the help.

Regards,

J

[Jordi Benet]

Hi,

just to add more information. My doubt comes from the 2 Cisco live sessions about DCI.

On this session it says that we must count 4 times the traffic passing through the DCI in order to know the DCI Distance:

https://www.ciscolive.com/online/connect/sessionDetail.ww?SESSION_ID=6623&backBtn=true

On this other session it says that we must count only 2 times:

https://www.ciscolive.com/online/connect/sessionDetail.ww?SESSION_ID=7775&backBtn=true

Who is right then? because the impact is huge.

Thank you.

Regards,

J

[Walter Dey]

https://www.ciscolive.com/online/connect/sessionDetail.ww?SESSION_ID=6623&backBtn=true

The whole discussion around round trip delay refers to the clasical fibre channel protocol

p 13 refers to a classical FC write operation, which requires 2 round trip

p 15 is a optimization: Cisco Write Acceleration feature on MDS Fabric; the local switch proxies the first RT, by sending a OK to the initiator; therefore, only one round trip