topic Re: How does the readAheadBlocks Oracle internal parameter affect the source database resources? in Qlik Replicate

How does the readAheadBlocks Oracle internal parameter affect the source database resources?

MoeE — Fri, 25 Sep 2026 05:07:18 GMT

Hi,

How does increasing the readAheadBlocks internal parameter on an Oracle source endpoint increase the memory usage on the Oracle server machine? Thanks

Regards,

Mohammed

Re: How does the readAheadBlocks Oracle internal parameter affect the source database resources?

sarahlauren0471 — Fri, 25 Sep 2026 07:44:11 GMT

Increasing readAheadBlocks can increase memory usage because the source endpoint may prefetch more blocks into memory for faster processing. With Oracle, a higher setting can therefore increase the amount of data held in buffers, especially when multiple tasks or connections are active at the same time. It’s usually better to increase it gradually while monitoring Oracle memory and workload rather than assuming a larger value will always improve performance. Visit here for more information.

Re: How does the readAheadBlocks Oracle internal parameter affect the source database resources?

SushilKumar — Fri, 25 Sep 2026 10:24:16 GMT

Hello @MoeE

The readAheadBlocks parameter controls how much Oracle redo data Qlik Replicate reads in each operation. The default is 100,000 blocks (~50 MB). Increasing it to 200,000–300,000 allows Replicate to read larger chunks and may help reduce CDC latency.

However, higher values can increase:

Network bandwidth usage, especially with multiple parallel tasks.
Source-side contention, as multiple tasks may read larger chunks from the same redo logs.

For example, increasing readAheadBlocks to 200,000 has helped reduce latency in some environments. However, the value should be balanced against available network and source database resources.

Regards,
Sushil kumar

Re: How does the readAheadBlocks Oracle internal parameter affect the source database resources?

MoeE — Mon, 28 Sep 2026 02:38:00 GMT

Hi Sarah,

I think the link you pasted here is incorrect

Reagrds,

Mohammed

Re: How does the readAheadBlocks Oracle internal parameter affect the source database resources?

MoeE — Mon, 28 Sep 2026 03:00:12 GMT

Hi Sushil,

Do you know if the memory usage on the source database or the Qlik Replicate server will increase?

Regards,

Mohammed

Re: How does the readAheadBlocks Oracle internal parameter affect the source database resources?

Bartosz567 — Tue, 29 Sep 2026 08:00:42 GMT

Hi Mohammed,

To clarify: the memory increase is on the Qlik Replicate server side, not the Oracle source machine.

readAheadBlocks controls how many blocks Replicate reads from the redo logs in one operation. That larger chunk of redo data is buffered in Replicate's own process memory for parsing. The Oracle server sees higher I/O and network transfer for the bigger read requests, but it is not the one allocating additional memory to hold the prefetched blocks.

So: bigger readAheadBlocks → more Qlik Replicate server RAM used. Source Oracle memory usage should remain largely unaffected, though you may observe brief spikes in source I/O as it fulfills the larger read requests.

Regards.

Re: How does the readAheadBlocks Oracle internal parameter affect the source database resources?

MoeE — Wed, 30 Sep 2026 01:06:29 GMT

Hi Bartosz567,

Thanks for the response. Are you aware of what the resource usage of the asmUsePLSQLArray internal parameter will be on a source Oracle DB in terms of memory usage?

Here is my logic and im not sure if it is correct. So this internal parameter buffers 50x32kb reads per ASM read thread. So that's ~ 1.6mb per ASM read threads. At 8 read threads, that's ~12.5mb of memory usage on the source oracle database. Is my logic correct or am I missing something? Thanks.

Regards,

Mohammed