[gpfsug-discuss] Write performances and filesystem size

Ivano Talamo Ivano.Talamo at psi.ch
Wed Nov 15 16:24:52 GMT 2017


Hello everybody,

together with my colleagues we are actually running some tests on a new 
DSS G220 system and we see some unexpected behaviour.

What we actually see is that write performances (we did not test read 
yet) decreases with the decrease of filesystem size.

I will not go into the details of the tests, but here are some numbers:

- with a filesystem using the full 1.2 PB space we get 14 GB/s as the 
sum of the disk activity on the two IO servers;
- with a filesystem using half of the space we get 10 GB/s;
- with a filesystem using 1/4 of the space we get 5 GB/s.

We also saw that performances are not affected by the vdisks layout, ie. 
taking the full space with one big vdisk or 2 half-size vdisks per RG 
gives the same performances.

To our understanding the IO should be spread evenly across all the 
pdisks in the declustered array, and looking at iostat all disks seem to 
be accessed. But so there must be some other element that affects 
performances.

Am I missing something? Is this an expected behaviour and someone has an 
explanation for this?

Thank you,
Ivano



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