PREEMPTIVE_OS_FLUSHFILEBUFFERS waits on Linux

Last week I blogged about Using the Wait Statistics Report in Azure Data Studio, using a Linux VM in Azure as an example host. As that was my first time running wait stats analysis on a Linux host, I found that the PREEMPTIVE_OS_FLUSHFILEBUFFERS waits on Linux will show up as the top wait on SQL Server 2017 CU6 and higher with default options.

The simple reason for this is that on some Linux servers where the underling I/O subsystem cannot guarantee durable writes when the power fails (i.e. it’s using a volatile write cache), a power failure could lead to data loss within SQL Server. To work around this, 2017 CU6 introduced forced-flush behavior for log and data writing, including things like backups and writes to FILESTREAM files. This means that the writes will be done as normal, and then a call is made to the Windows FlushFileBuffers function, which is translated via the SQLPAL/HostExtension wrapper on Linux into an fdatasync call. There’s a very deep explanation of all the file system internals in Bob Dorr’s excellent post SQL Server On Linux: Forced Unit Access (Fua) Internals.

If you know your I/O subsystem is resilient, you can disable this behavior, as described in KB 4131496.

However, as this will be by far the highest wait on many Linux systems, and as it’s benign, I’ve added it to the list of waits I ignore and updated the Server Reports extension for Azure Data Studio to filter it out as well (this is now live as v1.4).

Enjoy!

PS I also just added the VDI_CLIENT_OTHER wait to the ignore list as well (common from Availability Group seeding), and submitted GitHub changes to Server Reports yesterday, which will be v1.5.

2 thoughts on “PREEMPTIVE_OS_FLUSHFILEBUFFERS waits on Linux

  1. On a recent HammerDB TPC-C test we had to disable this behavior in order for SQL on Linux to perform as well as SQL on Windows. Leaving the now default CU6 behavior had a performance hit of 20%+. This was SQL Server running on an AWS EC2 instance with EBS storage.

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