Whats the difference between log file sync V/ log file parallel write

Whats the difference between log file sync V/ log file parallel write

Here's a simplified version of what's going on:

- Your session does "commit"

- It is now waiting on "log file sync"

- Now LGWR must flush the redo buffer to disk

- The I/O time for this will be "log file parallel write"

- Now LGWR is done, your session is notified

- Your session is back running again

Log file sync is typically caused by three things

1) committing takes too long

Check 'log file parallel write'. If that's a large percentage of the 'log file sync' time, then you might have an I/O performance issue. Redo writes should be expected to be 1ms or less on modern hardware.

2) committing too frequently

(Roughly speaking) when one person commits, "everyone" else wanting to commit waits on 'log file sync'. If your application commits excessively, this delay can mount up. But also note - this can mean that 'log file sync' is high on your list of waits, but not necessarily a problem that needs to be fixed.

3) resource pressure

If the server is overloaded (in particular on CPU), the lgwr process can struggle for resources, which slows it down...which slows all commits down.