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EMC SRDF Basics

December 3rd, 2008 No comments

Conceptually and operationally, SRDF is designed to work in a WAN/Internet/Cloud/SAN environment with multiple Symms involved, while Timefinder is local to a Symm, but performs the same functions.

The difference, SRDF can be performed without Geographic boundaries, while Timefinder is local. The following are various different forms of SRDF that can be used by a customer to perform SRDF operations. 

Synchronous mode

With Synchronous mode, the remote symm must have I/O in cache before the application receives the acknowledgement. Depending on distance where these Symmetrix machines are located, this may have a significant impact on performance. This form of SRDF is suggested to be implemented in a campus environment.

If you want to ensure that the data is replicated real time without dirty tracks from one symmetrix to the other, you might want to enable Domino effect. With Domino effect, your R1 devices will become not ready if the R2 devices cant be reached. 

Semi-synchronous mode

With Semi-synchronous mode, the I/O between the R1 and R2 devices are always out of sync. The application receives the acknowledgement from the first write I/O to the local cache. The second I/O isn’t acknowledged until the first is in the remote cache. This form of SRDF is faster than the previous mentioned Synchronous mode. 

Adaptive Copy-Write Pending

With Adaptive Copy-Write Pending, all the R2 volumes are copied over without the delay of acknowledgement from the application. With this mode, we can setup a skew parameter that will allow max number of dirty tracks. Once that number is reached, the system switches to a preconfigured mode like the semi-synchronous mode until the remote data is all synced. Once this is hit, SRDF is switched back to Adaptive Copy-Write Pending mode. 

SRDF Commands

December 2nd, 2008 1 comment

The following are SRDF Commands and what they are used for.

Composite SRDF commands

1. Failover:
1. Actions:
1. Write disables (WD) R1
2. Sets link to Not Ready (NR)
3. Write enables R2
2. Command:

symrdf -g ${group} failover

2. Update: Helps to speed up the failback operation by copying invalid tracks before write disabling any disks.
1. Actions:
1. Leaves service state as is.
2. Merges the tracks
3. Copies invalid tracks
2. Command:

symrdf -g ${group} update

3. Failback:
1. Actions:
1. Write disables R2
2. Suspends RDF link
3. Merges the disk tracks.
4. Resumes the link
5. Write enables R1
6. Copies the changed data
2. Command:

symrdf -g ${group} failback

4. Split: Leaves both R1 & R2 in write enabled state.
1. Actions:
1. Suspends the rdf link.
2. Write enables R2
2. Command:

symrdf -g ${group} split

5. Establish:
1. Actions:
1. Write disables R2
2. Suspends the rdf link
3. Copies data from R1 to R2
4. Resumes the rdf link.
2. Command:

symrdf -g ${group} [ -full ] establish

6. Restore: Copies data from R2 to R1
1. Actions:
1. Write disables both R1 & R2
2. Suspends the rdf link.
3. Merges the track tables
4. Resumes the rdf link.
5. Write enables R1
2. Command:

symrdf -g ${group} [ -full ] restore

Singular SRDF commands
1. Suspend: symrdf -g ${group} suspend
2. Resume: symrdf -g ${group} resume
3. Set mode: symrdf -g ${group} set mode sync
symrdf -g ${group} set domino on
symrdf -g ${group} set acp_disk skew 1000