TS7700 Synchronous Mode Copy Benefits

Is TS7700 Synchronous Mode Copy Just Another Replication Mode?

It seems awkward to still be talking about Synchronous Mode Copy in 2021 (almost 2022). Synchronous Mode Copy was introduced in 2015 with TS7700 R2.1 and has been in use by many environments since then. However, there are still customers using Immediate copies (also called RUN copies) or deferred copies for local clusters.

The two primary benefits to using Synchronous Mode Copy (Sync Mode) for local copies over Deferred Copies are:

• Reduce Your RPO (Recovery Point Objective)
• Avoid waiting for a job until the Rewind Unload (RUN) copy is produced

These two reasons are obvious, but a lesser-known benefit to using Synchronous Mode is a more efficient cache flow and therefore a more efficient utilization of the disk cache.

TS7700 Cache Flow and Disk Utilization

Immediate and Deferred copies read the data from cache to produce the copy. This is shown as “Outbound copy data rate” in the IntelliMagic Vision chart below. Synchronous Mode Copy does not need this additional read from cache because the write is directly sent to the secondary cluster during the original write. So, one type of reading from cache is not needed anymore using Synchronous Mode Copy.

Figure 1 below compares the cache flow from a cluster before (left side) and after (right side) a customer’s site switched from Immediate and Deferred Copies to Sync Mode.

Figure 1 – Replication data flows in and out of Cache

The workload at these two days was similar. As you can see, the Synchronous Mode Copy is reported as “Inbound Remote Write data rate” and is lower than the MB/s cache flow which was needed for Immediate and Deferred Copies in the week before.

Will This Reduction Also Lower My Average of The Maximum Disk Utilization?

The answer is: It Depends.

If the cache bandwidth was exhausted before, the likelihood is high that a process which was prolonged due to missing cache cycles before will now use the freed-up cycles. In that case you will not see a decrease in the average of the maximum disk utilization.

Also, for TS7700 with Tape Attach, the cycles may be re-used for premigration – and again the overall disk utilization may not be lower.

However, the benefit that other process will run smoother should be worth the small effort of changing the content of the management class.