InfoScale™ 9.0 Replication Administrator's Guide - AIX

Tunable parameters for the write buffer space on the Primary in a private disk group

The amount of buffer space that can be allocated within the operating system to handle incoming writes is defined by the tunable vol_rvio_maxpool_sz, which defaults to 128MB.

Figure: How VVR uses buffers between the Primary and Secondary shows the buffering process during a write operation.

Figure: How VVR uses buffers between the Primary and Secondary

If the available buffer space is not sufficient to process the write request, writes are held up. VVR must wait for current writes to complete and release the memory being used before processing new writes.

Furthermore, when the buffer space is low, VVR frees buffers early, requiring VVR to read back the write from the SRL.

Both these problems can be alleviated by increasing vol_rvio_maxpool_sz. By setting the vol_rvio_maxpool_sz to be large enough to hold the incoming writes, you can increase the number of concurrent writes and reduce the number of readbacks from the SRL. When decreasing the value of the vol_rvio_maxpool_sz tunable, stop all the RVGs on the system on which you are performing this operation.

When deciding whether or not a given write is freed early and read back later, VVR looks at the amount of buffer space available, and frees the write if the amount is below a threshold defined by the tunable vol_min_lowmem_sz. If this threshold is too low, it results in buffers being held for a long time. New writes cannot be performed because of lack of buffer space.

The vol_min_lowmem_sz tunable is 4MB (4194304 bytes).

You can raise the threshold by increasing the value of the tunable vol_min_lowmem_sz. It should be set to at least 3 x N x I, but not less than 520K, where N is the number of concurrent writes to replicated volumes, and I is the average I/O size, rounded up to 8 kilobytes. The vol_min_lowmem_sz tunable is auto-tunable and depending on the incoming writes, VVR will increase or decrease the tunable value. The value that you specify for the tunable, using the vxtune utility or the system-specific interface, will be used as an initial value and could change depending on the application write behavior.

Note that this tunable is used only when replicating in asynchronous mode because SRL is not read back when replicating in synchronous mode.

Use the vxrvg stats command to determine the maximum concurrency (N) and average write size (I).