How does fio determine I/O start/end with size, offset, and rw=write:-N together?

[CM-Chang]

I have a testing scenario with a single drive where I set the following parameters:
size=50%
bs=32M
rw=write:-(32M+32K)
Then I run tests with different offset values (e.g., 50%).
My expectation was that with offset=50%, fio would start writing backward from the 50% position toward 0%.
However, what I observed is that fio starts from the 100% position and writes backward.

I also tried other combinations, and here are the results I got:

size=50%, offset=20% → backward from 50%

size=50%, offset=45% → backward from 50%

size=50%, offset=70% → backward from 100%

size=60%, offset=20% → backward from 60%

size=60%, offset=45% → backward from 55%

size=60%, offset=70% → backward from 100%

My question is:
How exactly does fio determine the start and end positions for backward writes when size and offset are both specified with rw=write:-N?
Is there an internal rule or calculation that explains these results?

[sitsofe]

Hello @CM-Chang:

My expectation was that with offset=50%, fio would start writing backward from the 50% position toward 0%.

Your expectation is different to what is documented :-). If you read the documentation for offset does this make things any clearer? Let us know in a reply!

FYI:

• A blocksize of 32 Megabytes is huge and will likely be split up by the kernel as it makes its way down to lower layers...
• I don't think -(32M+32K) is a valid value for readwrite's :nr because nr isn't an int so arithmetic won't be done on it...