Symptoms
The following error log appears in /var/log/apache2/error.log in Debian-based or in /var/log/httpd/error_log on RHEL-based operating systems:
[Tue Oct 17 12:46:05.744609 2023] [pagespeed:error] [pid 12766:tid 140467601647360] [mod_pagespeed 1.13.35.2-0 @12766] Slow write operation on file /var/cache/mod_pagespeed/prop_page/https,3A/,2Finfo.example.com/www/delivery/afr.php,3Frefresh=10,26zoneid=1,26target=_blank,26cb=INSERT_RANDOM_NUMBER_HERE,26loc=https,3A/,2Fwww.example.com/_2L-ZMDIrHf,40Desktop,40beacon_cohort,.tempvYAllJ: 50.282ms; configure SlowFileLatencyUs to change thresholdn
[Tue Oct 03 10:32:41.965156 2017] [pagespeed:error] [pid 23476] [mod_pagespeed 1.12.34.2-0 @23476] Slow ReadFile operation on file /var/cache/mod_pagespeed/v3/example.com/https,3A/,2Fwww.example.com/forums,: 4039.37ms; configure SlowFileLatencyUs to change threshold
Cause
Slow disk I/O performance, which is regularly caused by a limitation for disk read and write processes or outdated disk-related hardware.
The hypervisor that resides on a level above your Plesk server or exists due to the hardware itself (for example, if the hardware of the disk that holds the /var/cache directory is either HDD, SSHD or is experiencing hardware issues).
Resolution
Increase the disk performance, for example by increasing the disk I/O limits set on the end of the hypervisor that manages the virtual server or by switching the server's hardware disks to faster ones (if you are using a HDD or SSHD drive, you need to switch it to SSD) and look for any hardware faults that are tied to the disk that contains the /var/cache directory.