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2021-04-23T01:11:07.784483Z
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pms(4): Limit maximum I/O size to 256KB instead of 1MB.

There is a weird limit of AGTIAPI_MAX_DMA_SEGS (128) S/G segments per
I/O since the initial driver import.  I don't know why it was added,
can only guess some hardware limitation, but in worst case it means
maximum I/O size of 508KB.  Respect it to be safe, rounding to 256KB.

MFC after:	1 week
Sponsored by:	iXsystems, Inc.

(cherry picked from commit 3e347834200b5d91a33384e696793e4ac20a44d4)

Git Hash:   39c37df826a73a8dd4d2d3e5909102969291170e
Git Author: mav@FreeBSD.org
END