What are the risks associated with increasing the Max Concurrent Writes setting?

Hey all! We’re running Syncthing on our Synology Nas moving a couple TB of data to and from our artists around the world. I was poking around the Advanced settings when I saw you can boost the amount of concurrent writes Syncthing is capable of. Seems to me that I could double or triple this from 2 to 4 or 6 and get much quicker transfers yeah? I’m sure this increase CPU usage so it would need to be monitored but it does seem that simple.

Am I missing something? I wanted to formally ask since it is an advanced setting and I don’t want to go flipping switches in there without being sure.


The discussion was inconclusive…

I think, based on absolutely no testing whatsoever, there would be little benefit to increasing it.

From my view solid state discs will write as fast as they can and queuing multiple files won’t change that. HDD are just more likely to thrash.

A lot of work is put into the firmware and caching mechanisms of storage drives and there’s no need to try and outsmart them.

Okay gotcha. So if you had four files downloading at the same time instead of two it wouldn’t improve things significantly/at all?

I would say that no-one knows for sure, at least not yet. You would need to run some actual benchmarks to measure whether there is any real difference or not.

Why do you think it would improve things?

The only case where it might is tons if tiny files.

If four files were downloading instead of two while the rest sit in the que doesn’t that bring those four files in faster?

I don’t see how number of files would affect the speed at which your disk is able to read (disks read blocks, they don’t even know about files) or the speed of your internet, which is usually the bottleneck.

It might have a benefit if you have tiny (sub 128k) files, and most likely have downside for anything bigger than 128k.

Max concurrent writes is specifically just about how many write requests we have outstanding towards the filesystem at any given point. If you have a disk system where each request is slow (spinning disks) but it can handle a lot of them concurrently (RAID of many spinning disks) then increasing this might be helpful. In most other cases it won’t. It doesn’t affect anything about how Syncthing requests data over the network.

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