HOWTO: Add note/exception on allow_mounted_write=

allow_mounted_write= is useful on Linux, but some platforms
just return some error on open(2) for mounted devices.

 # uname
 Linux
 # mkfs.ext4 /dev/loop0 > /dev/null
 mke2fs 1.43.3 (04-Sep-2016)
 # mount /dev/loop0 /mnt
 # mount | grep /dev/loop0
 /dev/loop0 on /mnt type ext4 (rw,relatime,seclabel,data=ordered)
 # fio --name=xxxxx --ioengine=sync --rw=write --bs=32k --size=1m --filename=/dev/loop0
 xxxxx: (g=0): rw=write, bs=32.0KiB-32.0KiB,32.0KiB-32.0KiB,32.0KiB-32.0KiB, ioengine=sync, iodepth=1
 fio-2.18-21-gca205
 fio: /dev/loop0 appears mounted, and 'allow_mounted_write' isn't set. Aborting.
 Run status group 0 (all jobs):
 # fio --name=xxxxx --ioengine=sync --rw=write --bs=32k --size=1m --filename=/dev/loop0 --allow_mounted_write=1 > /dev/null; echo $?
 0

 # uname
 FreeBSD
 # newfs /dev/da1 > /dev/null
 # mount /dev/da1 /mnt
 # mount | grep /dev/da1
 /dev/da1 on /mnt (ufs, local)
 # fio --name=xxxxx --ioengine=sync --rw=write --bs=32k --size=1m --filename=/dev/da1 --allow_mounted_write=1
 fio: this platform does not support process shared mutexes, forcing use of threads. Use the 'thread' option to get rid of this warning.
 xxxxx: (g=0): rw=write, bs=32.0KiB-32.0KiB,32.0KiB-32.0KiB,32.0KiB-32.0KiB, ioengine=sync, iodepth=1
 fio-2.18-21-gca205
 Starting 1 thread
 fio: failed opening chardev /dev/da1 for size check
 file:filesetup.c:654, func=open(/dev/da1), error=Operation not permitted
 fio: pid=0, err=1/file:filesetup.c:654, func=open(/dev/da1), error=Operation not permitted
 Run status group 0 (all jobs)

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/HOWTO b/HOWTO
index c2c65096bbd22461590c502aba6771b26598f672..e376ea5078ae3fdc0bf1d3fa4690987c728b2e60 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -886,7 +886,8 @@ Target file/device
        If this isn't set, fio will abort jobs that are destructive (e.g. that write)
        to what appears to be a mounted device or partition. This should help catch
        creating inadvertently destructive tests, not realizing that the test will
        If this isn't set, fio will abort jobs that are destructive (e.g. that write)
        to what appears to be a mounted device or partition. This should help catch
        creating inadvertently destructive tests, not realizing that the test will

-       destroy data on the mounted file system. Default: false.
+       destroy data on the mounted file system. Note that some platforms don't allow
+       writing against a mounted device regardless of this option. Default: false.

 
 .. option:: pre_read=bool
 
 
 .. option:: pre_read=bool