rhel7: Sync commands cause a process short hang

Issue

The process communicating with the DB server had a short hang for 67 seconds. The system IO load at the time were high (dirty and writeback were large), and the sync commands were executed multiple times.

In addition, the sync commands were operating frequently. In our test environment, we confirmed that processes have short hang under the following conditions.

• Execute dd to xfs filesystem (increase dirty and writeback)
• Execute multiple sync commands

When we change the filesystem from xfs to ext4, the symptom does not occur, so we think that there may be a problem of xfs.