[lustre-discuss] How does one process write to multiple OSTs

Oliver Mangold Oliver.Mangold at EMEA.NEC.COM
Wed Jul 1 23:40:32 PDT 2015

On 02.07.2015 05:59, Prakrati.Agrawal at shell.com<mailto:Prakrati.Agrawal at shell.com> wrote:

I have two very basic doubts.

1.       If I have only one process writing to multiple OSTs, is it still a parallel write or does it become sequential as only one process is writing?

In principle it should still be parallel, but performance depends on many factors, like e.g. the access patterns and client cache settings.

2.       Does MPI communication fabric interferes with file I/O?

Performance-wise certainly. Concerning correctness, no. Many people are doing that without problems.
I have a basic doubt:
If I have a directory whose stripe size is default and stripe count is set to 1. I have thousand files in that directory.
Total number of OSTs in my Lustre file system is say 8. Then how are those thousand files written to the OSTs?
Is it that first one OST is filled up with as many files as possible and then next OST is targeted or is it like file 1 to OST 1, file 2 to OST 2 and so on and then it comes back from file 13 to OST1.

If the stripe index of the directory is set to -1, then the files are distributed round-robin to the OSTs.
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