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
        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
 
 
 .. 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.
 
        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
 .. option:: verify_offset=int
 
        Swap the verification header with data somewhere else in the block before