Question regarding relevance of syncer(4) in the context of ZFS

[Konstantin Belousov]

On Fri, Aug 10, 2018 at 05:28:11PM +0300, Paul wrote:
> Hello team,
> 
> 
> If my understanding is correct then ZFS does not need to be pushed around and being told when to sync data to devices.
> It is perfectly capable of keeping data consistent and synchronized according to configured options.
> 
> We even disable 'sync' option of file system that we use. But unfortunately we see a constant and periodical spikes 
> of load on our servers that are directly related to wake-ups of 'syncer' kernel daemon. Is it safe to assume that
> 'syncer' is not necessary on configurations that only use ZFS, and no other file systems? And hence, is it safe
> to assume that setting all of 'kern.filedelay', 'kern.dirdelay' and 'kern.metadelay' to some large values will not 
> compromise system's durability?

One of the job of the syncer is to convert dirty user-mapped pages into
the filesystem-specific write requests.  Without syncer touching the mount
point, corresponding pages could linger forever, or at least until a memory
pressure causes pagedaemon to clean them.