random error and slowness

[hp2]

Setup: OLS 1.7.17, PHP 5.6 and MariaDB 10.4. Server has 6 cores CPU with 16GB RAM, running on SSD disk array.
(Yes, I know PHP and MariaDB are old, but customer's code is not ready for newer versions)

The situation:

1. Slowness:
- it's happening randomly, almost everyday. Each time it's only around 1-2 minutes, then fast again. When it's happening, the server load is low, around 1, has plenty of RAM.

2. Data error:
- also happening randomly. Only this returns:

gzdecode(): data erroron line $config = gzdecode( $config );

Because both happens randomly, and even I tried to check log realtime when it's happening, or comparing the timestamp, I could not find out any reason yet.
It's not because of OLS, it's happening with Apache too. But Apache is too slow, that's why we moved to OLS. Besides, somehow, with OLS, the above data error happens more frequently.

Is there any way to debug?
Thank you.

 

[Cold-Egg]

Any security service or plugin for this site? How's the DB performance during the slowness issue happening? Any io issue?

 

[hp2]

No plugin. No mod_security.
When the slowness happens, I don't think that IO is an issue, unless I am misunderstanding.

# vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
1 0 524284 2045196 0 9191700 0 0 143 390 1 2 3 1 96 0 0