doc: add text about possibly verify errors with norandommap

[fio.git] / fio.1

diff --git a/fio.1 b/fio.1
index ce3585a43a6660bdda2e45000bea7e9a9825c6ec..6d2eba67b1419d8d3e60b886c83bde9d87018fa0 100644 (file)
--- a/fio.1
+++ b/fio.1
@@ -68,12 +68,11 @@ available ioengines.
 Convert \fIjobfile\fR to a set of command\-line options.
 .TP
 .BI \-\-readonly
-Turn on safety read\-only checks, preventing writes. The \fB\-\-readonly\fR
+Turn on safety read\-only checks, preventing writes and trims. The \fB\-\-readonly\fR
 option is an extra safety guard to prevent users from accidentally starting
-a write workload when that is not desired. Fio will only write if
-`rw=write/randwrite/rw/randrw' is given. This extra safety net can be used
-as an extra precaution as \fB\-\-readonly\fR will also enable a write check in
-the I/O engine core to prevent writes due to unknown user space bug(s).
+a write or trim workload when that is not desired. Fio will only modify the
+device under test if `rw=write/randwrite/rw/randrw/trim/randtrim/trimwrite'
+is given. This safety net can be used as an extra precaution.
 .TP
 .BI \-\-eta \fR=\fPwhen
 Specifies when real\-time ETA estimate should be printed. \fIwhen\fR may
@@ -1121,7 +1120,9 @@ at past I/O history. This means that some blocks may not be read or written,
 and that some blocks may be read/written more than once. If this option is
 used with \fBverify\fR and multiple blocksizes (via \fBbsrange\fR),
 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.
 .TP
 .BI softrandommap \fR=\fPbool
 See \fBnorandommap\fR. If fio runs with the random block map enabled and
@@ -1227,7 +1228,7 @@ If you want a workload that has 50% 2k reads and 50% 4k reads, while having
 90% 4k writes and 10% 8k writes, you would specify:
 .RS
 .P
-bssplit=2k/50:4k/50,4k/90,8k/10
+bssplit=2k/50:4k/50,4k/90:8k/10
 .RE
 .P
 Fio supports defining up to 64 different weights for each data direction.
@@ -2371,6 +2372,11 @@ that the written data is also correctly read back. If the data direction
 given is a read or random read, fio will assume that it should verify a
 previously written file. If the data direction includes any form of write,
 the verify will be of the newly written data.
+.P
+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.
 .RE
 .TP
 .BI verify_offset \fR=\fPint