Re: Changing default value of wal_sync_method to open_datasync onLinux

On 2018-02-20 01:56:17 +0000, Tsunakawa, Takayuki wrote:
> Disabling the filesystem barrier is a valid tuning method as the PG manual says:

I don't think it says that:

> https://www.postgresql.org/docs/devel/static/wal-reliability.html
>
> [Excerpt]
> Recent SATA drives (those following ATAPI-6 or later) offer a drive cache flush command (FLUSH CACHE EXT), while SCSI
driveshave long supported a similar command SYNCHRONIZE CACHE. These commands are not directly accessible to
PostgreSQL,but some file systems (e.g., ZFS, ext4) can use them to flush data to the platters on write-back-enabled
drives.Unfortunately, such file systems behave suboptimally when combined with battery-backup unit (BBU) disk
controllers.In such setups, the synchronize command forces all data from the controller cache to the disks, eliminating
muchof the benefit of the BBU. You can run the pg_test_fsync program to see if you are affected. If you are affected,
theperformance benefits of the BBU can be regained by turning off write barriers in the file system or reconfiguring
thedisk controller, if that is an option. If write barriers are turned off, make sure the battery remains functional; a
faultybattery can potentially lead to data loss. Hopefully file system and disk controller designers will eventually
addressthis suboptimal behavior. 

Note it's only valid if running with a BBU. In which case the
performance measurements you're doing aren't particularly meaningful
anyway, as you'd test BBU performance rather than disk performance.


Greetings,

Andres Freund