Say my program attempts a read of a byte in a file on a ZFS filesystem. ZFS can locate a copy of the necessary block, but cannot locate any copy with a valid checksum (they’re all corrupted, or the only disks present have corrupted copies). What does my program see, in terms of the return value from the read, and the byte it tried to read? And is there a way to influence the behavior (under Solaris, or any other ZFS-implementing OS), that is, force failure, or force success, with potentially corrupt data?
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EIO is indeed the only answer with current ZFS implementations.
An open ZFS “bug” asks for some way to read corrupted data:
http://bugs.opensolaris.org/bugdatabase/printableBug.do?bug_id=6186106
I believe this is already doable using the undocumented but open source zdb utility.
Have a look at http://www.cuddletech.com/blog/pivot/entry.php?id=980 for explanations about how to dump a file content using zdb -R option and “r” flag.