Bug #108793 Free buffers difference between 5.7/8.0
Submitted: 16 Oct 2022 19:16 Modified: 17 Oct 2022 17:13
Reporter: Charles Thompson Email Updates:
Status: Not a Bug Impact on me:
Category:MySQL Server Severity:S3 (Non-critical)
Version:8.0 OS:Linux
Assigned to: CPU Architecture:Any

[16 Oct 2022 19:16] Charles Thompson

I've been looking at "Free buffers" from SHOW ENGINE INNODB STATUS, and have noticed there's a big difference between 5.7/8.0. The variable innodb_lru_scan_depth is set to 1024. Is this intended for there to be such a big difference in Free buffers between the major versions?

From 5.7:
Free buffers   	8187
Free buffers   	1024
Free buffers   	1023
Free buffers   	1022
Free buffers   	1020
Free buffers   	1024
Free buffers   	1025
Free buffers   	1025
Free buffers   	1024

From 8.0:
Free buffers       15186057
Free buffers       1904844
Free buffers       1902359
Free buffers       1902475
Free buffers       1890803
Free buffers       1885421
Free buffers       1895707
Free buffers       1902973
Free buffers       1901475

How to repeat:
Run SHOW ENGINE INNODB STATUS on both 5.7/8.0 versions on a server that's been running for some time
[17 Oct 2022 12:19] MySQL Verification Team
Hi Mr. Thompson,

Thank you for your report.

However, it is not a bug.

Organisation and allocation of the buffers for InnoDB storage engine differs significantly between 5.7 and 8.0.

Hence, in short, this is expected behaviour.

Not a bug.
[17 Oct 2022 16:44] Charles Thompson

Thanks for confirming it isn't a bug. Is there any documentation as to what changed for it to behave so differently?
[17 Oct 2022 17:13] Charles Thompson
With how large the Free buffers is now, it seems like the value could be in bytes instead of pages for 8.0. The docs still lists it as pages though, so maybe it wasn't updated?
[18 Oct 2022 11:30] MySQL Verification Team
Hi Mr. Thompson,

Unfortunately, our manual for the internal functioning of our software is not maintained for a quite long time.

There is some documentation on how each feature was designed, but those WorkLogs are strictly for the internal use.