[LU-3440] ftello() system call not grok'ing with expected file position location

Related — Mon, 10 Jun 2013 15:22:03 +0000

We have an external chemistry application (GROMACS) which relies on the ftello() system call during it's checkpointing mechanism. On our Lustre 2.1.5 system, we see evidence that fello() does not report to be at the current EOF after multiple writes. To demonstrate, attached is a small reproducer attempting to mimic the usage within the application which iterates writing the same data and compares the results of ftello() immediately after the write (for which the app is assuming to now be at current eof) versus the the result from ftello() after a seek to SEEK_END has been performed.

On Lustre 2.1.5, these seem to deviate after the first write iteration on our system with the attached. We do not see this on Lustre 1.8.6 or a vanilla file system like ext3.

Below is example output from the reproducer on 3 filesystems. The only outlier below is in the first run on 2.1.5 during iteration #1. The app is expecting to be at an offset of 5120 after the second write, but ftello() reports an offset of 6656.

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Running in Lustre 2.1.5

staff$ ./a.out 2 512
size of the problem: 2 iters: 512

iter = 0
off_t offset before write to file: 0
off_t offset after write to file: 2560
off_t offset after seek: 2560

iter = 1
off_t offset before write to file: 2560
off_t offset after write to file: 6656
off_t offset after seek: 5120

-----------------------------------------------
Running in Lustre 1.8.6

./a.out 2 512
size of the problem: 2 iters: 512

iter = 0
off_t offset before write to file: 0
off_t offset after write to file: 2560
off_t offset after seek: 2560

iter = 1
off_t offset before write to file: 2560
off_t offset after write to file: 5120
off_t offset after seek: 5120

-----------------------------------------------
Running in vanilla ext3:

staff$ ./a.out 2 512
size of the problem: 2 iters: 512

iter = 0
off_t offset before write to file: 0
off_t offset after write to file: 2560
off_t offset after seek: 2560

iter = 1
off_t offset before write to file: 2560
off_t offset after write to file: 5120
off_t offset after seek: 5120
-----------------------------------------------

Whamcloud Community JIRA

[LU-3440] ftello() system call not grok'ing with expected file position location