[lustre-discuss] poor performance on reading small files

[Dilger, Andreas]

On Jul 29, 2016, at 03:33, Oliver Mangold <Oliver.Mangold at EMEA.NEC.COM> wrote:
> 
> On 29.07.2016 04:19, Riccardo Veraldi wrote:
>> 
>> I am using lustre on ZFS.
>> 
>> While write performances are excellent also on smaller files, I find 
>> there is a drop down in performance
>> on reading 20KB files. Performance can go as low as 200MB/sec or even 
>> less.
> 
> Getting 200 MB/s with 20kB files means you have to do 10000 metadata 
> ops/s. Don't want to say it is impossible to get more than that, but at 
> least with MDT on ZFS this doesn't sound bad either. Did you run an 
> mdtest on your system? Maybe some serious tuning of MD performance is in 
> order.

I'd agree with Oliver that getting 200MB/s with 20KB files is not too bad.
Are you using HDDs or SSDs for the MDT and OST devices?  If using HDDs,
are you using SSD L2ARC to allow the metadata and file data be cached in
L2ARC, and allowing enough time for L2ARC to be warmed up?

Are you using TCP or IB networking?  If using TCP then there is a lower
limit on the number of RPCs that can be handled compared to IB.

Cheers, Andreas