[lustre-discuss] Doubt about how a file is stored on the OSTs

[Prakrati.Agrawal at shell.com]

Hi,

I am doing some performance benchmarking on Lustre file system. To understand my results, I wanted to know how a file is written on the OSTs.
Following is what I am doing:
I have a file of 64 GB to be written
number of ranks is 64
number of nodes is 4
stripe count is 4
stripe size is 1GB

Let the 4 OSTs be OST1, OST2, OST3, and OST4.
Let the nodes be N1, N2, N3 and N4.
So each rank is writing 1 GB to 1 of the 4 OSTs.
What I want to know is that, since 16 ranks are writing 1GB from say N1, are all those ranks writing to OST1 only or it might be the case that out of the 16 ranks, some are writing to OST1, some to OST2 and so on.

Also, if I increase my stripe count i.e number of OSTs to total number of OSTs, but each rank is still writing 1GB and total ranks are 64, why does the performance degrade?

Thanks,
Prakrati
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