EMC: SRDF modes

Title:EMC: SRDF modes
Author:Douglas O’Leary <dkoleary@olearycomputers.com>
Description:EMC: SRDF modes
Date created:06/2001
Date updated:07/2005
Disclaimer:Standard: Use the information that follows at your own risk. If you screw up a system, don’t blame it on me...

Conceptually, even operationally, SRDF is very similar to Timefinder. About the only difference is that SRDF works across Symms; while Timefinder works internally to one Symm.

That difference, intersymm vs intrasym, means that SRDF operations can cover quite a bit of ground geographically. With the advent of geographically separated symms, the integrity of the data from one symm to the other becomes a concern. EMC has a number of operational modes in which the SRDF operates. The choice between these operational modes is a balancing act between how quickly the calling application gets an acknowledgement back versus how sure you need to be that the data has been received on the remote symm.

Synchronous mode

Synchronous mode basically means that the remote symm must have the I/O in cache before the calling application receives the acknowledgement. Depending on distance between symms, this may have a significant impact on performance which is the main reason that EMC suggests this set up in a campus (damn near colocated) environment only.

If you’re particularly paranoid about ensuring data on one symm is on the other, you can enable the Domino effect (I think you’re supposed to be hearing suspense music in the background right about now...). Basically, the domino effect ensures that the R1 devices will become “not ready” if the R2 devices can’t be reached for any reason - effectively shutting down the filesystem(s) untilthe problem can be resolved.

Semi-synchronous mode

In semi-synchronous mode, the R2 devices are one (or less) write I/O out of sync with their R1 device counterparts. The application gets the acknowledgement as soon as the first write I/O gets to the local cache. The second I/O isn’t acknowledged until the first is in the remote cache. This should speed up the application over the synchronous mode. It does, however, mean that your data might be a bit out of sync with the local symm.

Adaptive Copy-Write Pending

This mode copies data over to the R2 volumes as quickly as it can; however, doesn’t delay the acknowledgement to the application. This mode is useful where some data loss is permissable and local performance is paramount.

There’s a configurable skew parameter that lists the maximum allowable dirty tracks. Once that number of pending I/Os is reached, the system switches to the predetermined mode (probably semi-synchronous) until the remote symm catches up. At that point, it switches back to adaptive copy-write pending mode.