blktrace: update blkiomon doc

Fixup for 499398.

Description of problem:

blkiomon does not understand the output of blktrace when
working with logical volume device (it is quiet, while
working with physical device it prints IO statistics as
expected).

BUG (or design feature?):
/dev/dm-* and /dev/md* don't see BLK_TC_COMPLETE actions:

        /* we need an older D trace and a younger C trace */
        if (t_old->bit.action & BLK_TC_ACT(BLK_TC_ISSUE) &&
            t_young->bit.action & BLK_TC_ACT(BLK_TC_COMPLETE)) {
                /* matching D and C traces - update statistics */
                match++;
                blkiomon_account(&t_old->bit, &t_young->bit);
                blkiomon_free_trace(t_stored);
                return t;
        }

Possible solution:
Update documentation.

Signed-off-by: Edward Shishkin <edward@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/doc/blkiomon.8 b/doc/blkiomon.8
index dbed0df..aa5ece2 100644
--- a/doc/blkiomon.8
+++ b/doc/blkiomon.8
@@ -17,6 +17,8 @@ blkiomon is a block device I/O monitor. It periodically generates per-device
 request size and request latency statistics from blktrace data. It provides
 histograms as well as data that can be used to calculate min, max, average
 and variance. For this purpose, it consumes D and C traces read from stdin.
+Note, that this doesn't work for logical volumes, as high-level drivers
+don't see the completion of the events (C).
 
 There are options for binary output and human-readable output to files and
 stdout. Output to a message queue is supported as well.