docs: update HOWTO and manpage for sg trim support

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index e1399b4a101925d94527542e4c5f8f696668912b..804d93e4c578bfdb7d47058d177cce34420bb380 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -991,13 +991,15 @@ I/O type
                **write**
                                Sequential writes.
                **trim**
-                               Sequential trims (Linux block devices only).
+                               Sequential trims (Linux block devices and SCSI
+                               character devices only).
                **randread**
                                Random reads.
                **randwrite**
                                Random writes.
                **randtrim**
-                               Random trims (Linux block devices only).
+                               Random trims (Linux block devices and SCSI
+                               character devices only).
                **rw,readwrite**
                                Sequential mixed reads and writes.
                **randrw**
@@ -1329,7 +1331,9 @@ 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
 
@@ -1746,7 +1750,7 @@ I/O engine
                        ioctl, or if the target is an sg character device we use
                        :manpage:`read(2)` and :manpage:`write(2)` for asynchronous
                        I/O. Requires :option:`filename` option to specify either block or
-                       character devices.
+                       character devices. This engine supports trim operations.
                        The sg engine includes engine specific options.
 
                **null**
@@ -2080,6 +2084,7 @@ with the caveat that when used on the command line, they must come after the
        the force unit access (fua) flag. Default is 0.
 
 .. option:: sg_write_mode=str : [sg]
+
        Specify the type of write commands to issue. This option can take three values:
 
        **write**
@@ -2663,6 +2668,11 @@ 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