Re: Gluster very poor performance when copying small files (1x (2+1) = 3, SSD)

[Sam McLeod <mailinglists@xxxxxxxxxxx>]

Hi Raghavendra,

On 20 Mar 2018, at 1:55 pm, Raghavendra Gowdappa <rgowdapp@xxxxxxxxxx> wrote:

Aggregating large number of small writes by write-behind into large writes has been merged on master:

https://github.com/gluster/glusterfs/issues/364

Would like to know whether it helps for this usecase. Note that its not part of any release yet. So you've to build and install from repo.

Sounds interesting, not too keen to build packages at the moment but I've added myself as a watcher to that issue on Github and once it's in a 3.x release I'll try it and let you know.

Another suggestion is to run tests with turning off option performance.write-behind-trickling-writes.

# gluster volume set <volname> performance.write-behind-trickling-writes off

A word of caution though is if your files are too small, these suggestions may not have much impact.

I'm looking for documentation on this option but all I could really find is in the source for write-behind.c:

if is enabled (which it is), do not hold back writes if there are no outstanding requests.

and a note on aggregate-size stating that 

"aggregation won't happen if performance.write-behind-trickling-writes is turned on"

What are the potentially negative performance impacts of disabling this?

--
Sam McLeod (protoporpoise on IRC)

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