doc: add text about possibly verify errors with norandommap

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index e1399b4a101925d94527542e4c5f8f696668912b..70eed2805c2d96811d4238afaed645f4f3a10e45 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -1329,7 +1329,9 @@ I/O type
        and that some blocks may be read/written more than once. If this option is
        used with :option:`verify` and multiple blocksizes (via :option:`bsrange`),
        only intact blocks are verified, i.e., partially-overwritten blocks are
-       ignored.
+       ignored.  With an async I/O engine and an I/O depth > 1, it is possible for
+       the same block to be overwritten, which can cause verification errors.  Either
+       do not use norandommap in this case, or also use the lfsr random generator.
 
 .. option:: softrandommap=bool
 
@@ -2663,6 +2665,11 @@ Verification
        previously written file. If the data direction includes any form of write,
        the verify will be of the newly written data.
 
+       To avoid false verification errors, do not use the norandommap option when
+       verifying data with async I/O engines and I/O depths > 1.  Or use the
+       norandommap and the lfsr random generator together to avoid writing to the
+       same offset with muliple outstanding I/Os.
+
 .. option:: verify_offset=int
 
        Swap the verification header with data somewhere else in the block before