[PATCH] blktrace: add FLUSH/FUA support

[PATCH] blktrace: add FLUSH/FUA support

[Namhyung Kim]

Add FLUSH/FUA support to blktrace. As FLUSH precedes WRITE and/or
FUA follows WRITE, use the same 'F' flag for both cases and
distinguish them by their (relative) position. The end results
look like (other flags might be shown also):

 - WRITE:            W
 - WRITE_FLUSH:      FW
 - WRITE_FUA:        WF
 - WRITE_FLUSH_FUA:  FWF

Note that BLK_TC_FLUSH should be the last one due to MASC_TC_BIT().
Otherwise it will cause unpleasant result because __REQ_FLUSH (23)
would be greater than ilog2(BLK_TC_FLUSH) + BLK_TC_SHIFT (16) so the
negative value. __REQ_FUA (12) doesn't have this problem.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
---
 include/linux/blktrace_api.h |    5 +++--
 include/trace/events/block.h |   18 +++++++++---------
 kernel/trace/blktrace.c      |   21 ++++++++++++++++-----
 3 files changed, 28 insertions(+), 16 deletions(-)

diff --git a/include/linux/blktrace_api.h b/include/linux/blktrace_api.h
index b22fb0d3db0f..05a688648bef 100644
--- a/include/linux/blktrace_api.h
+++ b/include/linux/blktrace_api.h
@@ -14,7 +14,7 @@
 enum blktrace_cat {
 	BLK_TC_READ	= 1 << 0,	/* reads */
 	BLK_TC_WRITE	= 1 << 1,	/* writes */
-	BLK_TC_BARRIER	= 1 << 2,	/* barrier */
+	BLK_TC_FUA	= 1 << 2,	/* fua requests */
 	BLK_TC_SYNC	= 1 << 3,	/* sync IO */
 	BLK_TC_SYNCIO	= BLK_TC_SYNC,
 	BLK_TC_QUEUE	= 1 << 4,	/* queueing/merging */
@@ -28,8 +28,9 @@ enum blktrace_cat {
 	BLK_TC_META	= 1 << 12,	/* metadata */
 	BLK_TC_DISCARD	= 1 << 13,	/* discard requests */
 	BLK_TC_DRV_DATA	= 1 << 14,	/* binary per-driver data */
+	BLK_TC_FLUSH	= 1 << 15,	/* flush requests */
 
-	BLK_TC_END	= 1 << 15,	/* only 16-bits, reminder */
+	BLK_TC_END	= 1 << 15,	/* we've run out of bits! */
 };
 
 #define BLK_TC_SHIFT		(16)
diff --git a/include/trace/events/block.h b/include/trace/events/block.h
index bf366547da25..f21fea24216d 100644
--- a/include/trace/events/block.h
+++ b/include/trace/events/block.h
@@ -19,7 +19,7 @@ DECLARE_EVENT_CLASS(block_rq_with_error,
 		__field(  sector_t,	sector			)
 		__field(  unsigned int,	nr_sector		)
 		__field(  int,		errors			)
-		__array(  char,		rwbs,	6		)
+		__array(  char,		rwbs,	8		)
 		__dynamic_array( char,	cmd,	blk_cmd_buf_len(rq)	)
 	),
 
@@ -104,7 +104,7 @@ DECLARE_EVENT_CLASS(block_rq,
 		__field(  sector_t,	sector			)
 		__field(  unsigned int,	nr_sector		)
 		__field(  unsigned int,	bytes			)
-		__array(  char,		rwbs,	6		)
+		__array(  char,		rwbs,	8		)
 		__array(  char,         comm,   TASK_COMM_LEN   )
 		__dynamic_array( char,	cmd,	blk_cmd_buf_len(rq)	)
 	),
@@ -183,7 +183,7 @@ TRACE_EVENT(block_bio_bounce,
 		__field( dev_t,		dev			)
 		__field( sector_t,	sector			)
 		__field( unsigned int,	nr_sector		)
-		__array( char,		rwbs,	6		)
+		__array( char,		rwbs,	8		)
 		__array( char,		comm,	TASK_COMM_LEN	)
 	),
 
@@ -222,7 +222,7 @@ TRACE_EVENT(block_bio_complete,
 		__field( sector_t,	sector		)
 		__field( unsigned,	nr_sector	)
 		__field( int,		error		)
-		__array( char,		rwbs,	6	)
+		__array( char,		rwbs,	8	)
 	),
 
 	TP_fast_assign(
@@ -249,7 +249,7 @@ DECLARE_EVENT_CLASS(block_bio,
 		__field( dev_t,		dev			)
 		__field( sector_t,	sector			)
 		__field( unsigned int,	nr_sector		)
-		__array( char,		rwbs,	6		)
+		__array( char,		rwbs,	8		)
 		__array( char,		comm,	TASK_COMM_LEN	)
 	),
 
@@ -321,7 +321,7 @@ DECLARE_EVENT_CLASS(block_get_rq,
 		__field( dev_t,		dev			)
 		__field( sector_t,	sector			)
 		__field( unsigned int,	nr_sector		)
-		__array( char,		rwbs,	6		)
+		__array( char,		rwbs,	8		)
 		__array( char,		comm,	TASK_COMM_LEN	)
         ),
 
@@ -456,7 +456,7 @@ TRACE_EVENT(block_split,
 		__field( dev_t,		dev				)
 		__field( sector_t,	sector				)
 		__field( sector_t,	new_sector			)
-		__array( char,		rwbs,		6		)
+		__array( char,		rwbs,		8		)
 		__array( char,		comm,		TASK_COMM_LEN	)
 	),
 
@@ -498,7 +498,7 @@ TRACE_EVENT(block_bio_remap,
 		__field( unsigned int,	nr_sector	)
 		__field( dev_t,		old_dev		)
 		__field( sector_t,	old_sector	)
-		__array( char,		rwbs,	6	)
+		__array( char,		rwbs,	8	)
 	),
 
 	TP_fast_assign(
@@ -542,7 +542,7 @@ TRACE_EVENT(block_rq_remap,
 		__field( unsigned int,	nr_sector	)
 		__field( dev_t,		old_dev		)
 		__field( sector_t,	old_sector	)
-		__array( char,		rwbs,	6	)
+		__array( char,		rwbs,	8	)
 	),
 
 	TP_fast_assign(
diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 6957aa298dfa..f5deb6f49e76 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -206,6 +206,8 @@ static void __blk_add_trace(struct blk_trace *bt, sector_t sector, int bytes,
 	what |= MASK_TC_BIT(rw, RAHEAD);
 	what |= MASK_TC_BIT(rw, META);
 	what |= MASK_TC_BIT(rw, DISCARD);
+	what |= MASK_TC_BIT(rw, FLUSH);
+	what |= MASK_TC_BIT(rw, FUA);
 
 	pid = tsk->pid;
 	if (act_log_check(bt, what, sector, pid))
@@ -1054,6 +1056,9 @@ static void fill_rwbs(char *rwbs, const struct blk_io_trace *t)
 		goto out;
 	}
 
+	if (tc & BLK_TC_FLUSH)
+		rwbs[i++] = 'F';
+
 	if (tc & BLK_TC_DISCARD)
 		rwbs[i++] = 'D';
 	else if (tc & BLK_TC_WRITE)
@@ -1063,10 +1068,10 @@ static void fill_rwbs(char *rwbs, const struct blk_io_trace *t)
 	else
 		rwbs[i++] = 'N';
 
+	if (tc & BLK_TC_FUA)
+		rwbs[i++] = 'F';
 	if (tc & BLK_TC_AHEAD)
 		rwbs[i++] = 'A';
-	if (tc & BLK_TC_BARRIER)
-		rwbs[i++] = 'B';
 	if (tc & BLK_TC_SYNC)
 		rwbs[i++] = 'S';
 	if (tc & BLK_TC_META)
@@ -1132,7 +1137,7 @@ typedef int (blk_log_action_t) (struct trace_iterator *iter, const char *act);
 
 static int blk_log_action_classic(struct trace_iterator *iter, const char *act)
 {
-	char rwbs[6];
+	char rwbs[8];
 	unsigned long long ts  = iter->ts;
 	unsigned long nsec_rem = do_div(ts, NSEC_PER_SEC);
 	unsigned secs	       = (unsigned long)ts;
@@ -1148,7 +1153,7 @@ static int blk_log_action_classic(struct trace_iterator *iter, const char *act)
 
 static int blk_log_action(struct trace_iterator *iter, const char *act)
 {
-	char rwbs[6];
+	char rwbs[8];
 	const struct blk_io_trace *t = te_blk_io_trace(iter->ent);
 
 	fill_rwbs(rwbs, t);
@@ -1561,7 +1566,7 @@ static const struct {
 } mask_maps[] = {
 	{ BLK_TC_READ,		"read"		},
 	{ BLK_TC_WRITE,		"write"		},
-	{ BLK_TC_BARRIER,	"barrier"	},
+	{ BLK_TC_FUA,		"fua"		},
 	{ BLK_TC_SYNC,		"sync"		},
 	{ BLK_TC_QUEUE,		"queue"		},
 	{ BLK_TC_REQUEUE,	"requeue"	},
@@ -1573,6 +1578,7 @@ static const struct {
 	{ BLK_TC_META,		"meta"		},
 	{ BLK_TC_DISCARD,	"discard"	},
 	{ BLK_TC_DRV_DATA,	"drv_data"	},
+	{ BLK_TC_FLUSH,		"flush"		},
 };
 
 static int blk_trace_str2mask(const char *str)
@@ -1788,6 +1794,9 @@ void blk_fill_rwbs(char *rwbs, u32 rw, int bytes)
 {
 	int i = 0;
 
+	if (rw & REQ_FLUSH)
+		rwbs[i++] = 'F';
+
 	if (rw & WRITE)
 		rwbs[i++] = 'W';
 	else if (rw & REQ_DISCARD)
@@ -1797,6 +1806,8 @@ void blk_fill_rwbs(char *rwbs, u32 rw, int bytes)
 	else
 		rwbs[i++] = 'N';
 
+	if (rw & REQ_FUA)
+		rwbs[i++] = 'F';
 	if (rw & REQ_RAHEAD)
 		rwbs[i++] = 'A';
 	if (rw & REQ_SYNC)
-- 
1.7.5.2

Re: [PATCH] blktrace: add FLUSH/FUA support

[Steven Rostedt]

On Wed, 2011-06-01 at 17:38 +0900, Namhyung Kim wrote:
> Add FLUSH/FUA support to blktrace. As FLUSH precedes WRITE and/or
> FUA follows WRITE, use the same 'F' flag for both cases and
> distinguish them by their (relative) position. The end results
> look like (other flags might be shown also):
> 
>  - WRITE:            W
>  - WRITE_FLUSH:      FW
>  - WRITE_FUA:        WF
>  - WRITE_FLUSH_FUA:  FWF
> 
> Note that BLK_TC_FLUSH should be the last one due to MASC_TC_BIT().
> Otherwise it will cause unpleasant result because __REQ_FLUSH (23)
> would be greater than ilog2(BLK_TC_FLUSH) + BLK_TC_SHIFT (16) so the
> negative value. __REQ_FUA (12) doesn't have this problem.
> 
> Signed-off-by: Namhyung Kim <namhyung@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> ---
>  include/linux/blktrace_api.h |    5 +++--
>  include/trace/events/block.h |   18 +++++++++---------
>  kernel/trace/blktrace.c      |   21 ++++++++++++++++-----
>  3 files changed, 28 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/blktrace_api.h b/include/linux/blktrace_api.h
> index b22fb0d3db0f..05a688648bef 100644
> --- a/include/linux/blktrace_api.h
> +++ b/include/linux/blktrace_api.h
> @@ -14,7 +14,7 @@
>  enum blktrace_cat {
>  	BLK_TC_READ	= 1 << 0,	/* reads */
>  	BLK_TC_WRITE	= 1 << 1,	/* writes */
> -	BLK_TC_BARRIER	= 1 << 2,	/* barrier */
> +	BLK_TC_FUA	= 1 << 2,	/* fua requests */
>  	BLK_TC_SYNC	= 1 << 3,	/* sync IO */
>  	BLK_TC_SYNCIO	= BLK_TC_SYNC,
>  	BLK_TC_QUEUE	= 1 << 4,	/* queueing/merging */
> @@ -28,8 +28,9 @@ enum blktrace_cat {
>  	BLK_TC_META	= 1 << 12,	/* metadata */
>  	BLK_TC_DISCARD	= 1 << 13,	/* discard requests */
>  	BLK_TC_DRV_DATA	= 1 << 14,	/* binary per-driver data */
> +	BLK_TC_FLUSH	= 1 << 15,	/* flush requests */
>  
> -	BLK_TC_END	= 1 << 15,	/* only 16-bits, reminder */
> +	BLK_TC_END	= 1 << 15,	/* we've run out of bits! */
>  };
>  
>  #define BLK_TC_SHIFT		(16)
> diff --git a/include/trace/events/block.h b/include/trace/events/block.h
> index bf366547da25..f21fea24216d 100644
> --- a/include/trace/events/block.h
> +++ b/include/trace/events/block.h
> @@ -19,7 +19,7 @@ DECLARE_EVENT_CLASS(block_rq_with_error,
>  		__field(  sector_t,	sector			)
>  		__field(  unsigned int,	nr_sector		)
>  		__field(  int,		errors			)
> -		__array(  char,		rwbs,	6		)
> +		__array(  char,		rwbs,	8		)
>  		__dynamic_array( char,	cmd,	blk_cmd_buf_len(rq)	)
>  	),
>  
> @@ -104,7 +104,7 @@ DECLARE_EVENT_CLASS(block_rq,
>  		__field(  sector_t,	sector			)
>  		__field(  unsigned int,	nr_sector		)
>  		__field(  unsigned int,	bytes			)
> -		__array(  char,		rwbs,	6		)
> +		__array(  char,		rwbs,	8		)
>  		__array(  char,         comm,   TASK_COMM_LEN   )
>  		__dynamic_array( char,	cmd,	blk_cmd_buf_len(rq)	)
>  	),
> @@ -183,7 +183,7 @@ TRACE_EVENT(block_bio_bounce,
>  		__field( dev_t,		dev			)
>  		__field( sector_t,	sector			)
>  		__field( unsigned int,	nr_sector		)
> -		__array( char,		rwbs,	6		)
> +		__array( char,		rwbs,	8		)
>  		__array( char,		comm,	TASK_COMM_LEN	)
>  	),
>  


Wouldn't it be better to replace all these "6" and "8"'s with a macro?
Then it would be a heck of a lot easier to change in the future.

-- Steve

Re: [PATCH] blktrace: add FLUSH/FUA support

[Namhyung Kim]

2011-06-07 (화), 19:20 -0400, Steven Rostedt:
> On Wed, 2011-06-01 at 17:38 +0900, Namhyung Kim wrote:
> > diff --git a/include/trace/events/block.h b/include/trace/events/block.h
> > index bf366547da25..f21fea24216d 100644
> > --- a/include/trace/events/block.h
> > +++ b/include/trace/events/block.h
> > @@ -19,7 +19,7 @@ DECLARE_EVENT_CLASS(block_rq_with_error,
> >  		__field(  sector_t,	sector			)
> >  		__field(  unsigned int,	nr_sector		)
> >  		__field(  int,		errors			)
> > -		__array(  char,		rwbs,	6		)
> > +		__array(  char,		rwbs,	8		)
> >  		__dynamic_array( char,	cmd,	blk_cmd_buf_len(rq)	)
> >  	),
> >  
> > @@ -104,7 +104,7 @@ DECLARE_EVENT_CLASS(block_rq,
> >  		__field(  sector_t,	sector			)
> >  		__field(  unsigned int,	nr_sector		)
> >  		__field(  unsigned int,	bytes			)
> > -		__array(  char,		rwbs,	6		)
> > +		__array(  char,		rwbs,	8		)
> >  		__array(  char,         comm,   TASK_COMM_LEN   )
> >  		__dynamic_array( char,	cmd,	blk_cmd_buf_len(rq)	)
> >  	),
> > @@ -183,7 +183,7 @@ TRACE_EVENT(block_bio_bounce,
> >  		__field( dev_t,		dev			)
> >  		__field( sector_t,	sector			)
> >  		__field( unsigned int,	nr_sector		)
> > -		__array( char,		rwbs,	6		)
> > +		__array( char,		rwbs,	8		)
> >  		__array( char,		comm,	TASK_COMM_LEN	)
> >  	),
> >  
> 
> 
> Wouldn't it be better to replace all these "6" and "8"'s with a macro?
> Then it would be a heck of a lot easier to change in the future.
> 
> -- Steve
> 
> 

Will fix in the next version, Thanks.


-- 
Regards,
Namhyung Kim

Re: [PATCH] blktrace: add FLUSH/FUA support

[Jeff Moyer]

Hi,

Sorry, I don't have the original posting of this message, so I've just
cut-n-paste from the archives on lkml.org:
  https://lkml.org/lkml/2011/6/1/235

The proposal was this:

> Add FLUSH/FUA support to blktrace. As FLUSH precedes WRITE and/or
> FUA follows WRITE, use the same 'F' flag for both cases and
> distinguish them by their (relative) position. The end results
> look like (other flags might be shown also):
> 
>  - WRITE:            W
>  - WRITE_FLUSH:      FW
>  - WRITE_FUA:        WF
>  - WRITE_FLUSH_FUA:  FWF

I'm not sure I'll ever be able to keep that straight.  How about we use
'F' for FUA, since FUA is capitalized anyway, and use 'f' for flush?
Too subtle?

Next...

> @@ -14,7 +14,7 @@
>  enum blktrace_cat {
>  	BLK_TC_READ	= 1 << 0,	/* reads */
>  	BLK_TC_WRITE	= 1 << 1,	/* writes */
> -	BLK_TC_BARRIER	= 1 << 2,	/* barrier */
> +	BLK_TC_FUA	= 1 << 2,	/* fua requests */

I would prefer to replace BARRIER with FLUSH, as I think they are closer
relatives.  Doing it the way you've suggested would mean that older
blktrace user-space would report FUA as a Barrier.

Comments?  No matter what's agreed upon, we should get this in sooner
rather than later, as it's a big missing piece in trying to diagnose
performance issues!

Cheers,
Jeff

Re: [PATCH] blktrace: add FLUSH/FUA support

[Namhyung Kim]

2011-07-28 (목), 16:21 -0400, Jeff Moyer:
> Hi,
> 
> Sorry, I don't have the original posting of this message, so I've just
> cut-n-paste from the archives on lkml.org:
>   https://lkml.org/lkml/2011/6/1/235
> 

Hello, Jeff.

Thanks for finding and replying to this :)


> The proposal was this:
> 
> > Add FLUSH/FUA support to blktrace. As FLUSH precedes WRITE and/or
> > FUA follows WRITE, use the same 'F' flag for both cases and
> > distinguish them by their (relative) position. The end results
> > look like (other flags might be shown also):
> > 
> >  - WRITE:            W
> >  - WRITE_FLUSH:      FW
> >  - WRITE_FUA:        WF
> >  - WRITE_FLUSH_FUA:  FWF
> 
> I'm not sure I'll ever be able to keep that straight.  How about we use
> 'F' for FUA, since FUA is capitalized anyway, and use 'f' for flush?
> Too subtle?
> 

Either way is fine to me. Jens?


> Next...
> 
> > @@ -14,7 +14,7 @@
> >  enum blktrace_cat {
> >  	BLK_TC_READ	= 1 << 0,	/* reads */
> >  	BLK_TC_WRITE	= 1 << 1,	/* writes */
> > -	BLK_TC_BARRIER	= 1 << 2,	/* barrier */
> > +	BLK_TC_FUA	= 1 << 2,	/* fua requests */
> 
> I would prefer to replace BARRIER with FLUSH, as I think they are closer
> relatives.  Doing it the way you've suggested would mean that older
> blktrace user-space would report FUA as a Barrier.
> 

I thought about that too. But as I said in the changelog, it led to a
negative number at the rhs of MASC_TC_BIT calculation, so the end result
was not good.

In the meantime, I found that Matthew Wilcox posted a patch which
relocates some REQ_ flags to appropriate positions.

https://lkml.org/lkml/2011/6/2/324

With the patch, it seems ok to replace BARRIER with FLUSH. However it
looks like the patch isn't included into the tree yet.

BTW, I'm thinking about user-space again. I'm not sure it's ok if older
blktrace tool reports FLUSH/FUA as BARRIER. Actually I posted a patch
that treats FLUSH as BARRIER [1], and Jens and others commented we
should not do that.

To end that, I could leave BLK_TC_BARRIER as is, and add
BLK_TC_{FLUSH,FUA} at the end of blktrace_cat. But as we exhause space
in the 16-bit act_mask, it would require a substantial change.

Any thoughts?


> Comments?  No matter what's agreed upon, we should get this in sooner
> rather than later, as it's a big missing piece in trying to diagnose
> performance issues!
> 
> Cheers,
> Jeff

Thanks.

[1] https://lkml.org/lkml/2011/5/27/206


-- 
Regards,
Namhyung Kim

Re: [PATCH] blktrace: add FLUSH/FUA support

[Jeff Moyer]

Namhyung Kim <namhyung@gmail.com> writes:

> 2011-07-28 (목), 16:21 -0400, Jeff Moyer:
>> Hi,
>> 
>> Sorry, I don't have the original posting of this message, so I've just
>> cut-n-paste from the archives on lkml.org:
>>   https://lkml.org/lkml/2011/6/1/235
>> 
>
> Hello, Jeff.
>
> Thanks for finding and replying to this :)
>
>
>> The proposal was this:
>> 
>> > Add FLUSH/FUA support to blktrace. As FLUSH precedes WRITE and/or
>> > FUA follows WRITE, use the same 'F' flag for both cases and
>> > distinguish them by their (relative) position. The end results
>> > look like (other flags might be shown also):
>> > 
>> >  - WRITE:            W
>> >  - WRITE_FLUSH:      FW
>> >  - WRITE_FUA:        WF
>> >  - WRITE_FLUSH_FUA:  FWF
>> 
>> I'm not sure I'll ever be able to keep that straight.  How about we use
>> 'F' for FUA, since FUA is capitalized anyway, and use 'f' for flush?
>> Too subtle?
>> 
>
> Either way is fine to me. Jens?

OK, having read your [1] below, Jens' suggestion was to have:

Write:        W
Write Flush:  F
Write + FUA:  WF
Flush + FUA:  FF

That actually makes sense to me.

>> Next...
>> 
>> > @@ -14,7 +14,7 @@
>> >  enum blktrace_cat {
>> >  	BLK_TC_READ	= 1 << 0,	/* reads */
>> >  	BLK_TC_WRITE	= 1 << 1,	/* writes */
>> > -	BLK_TC_BARRIER	= 1 << 2,	/* barrier */
>> > +	BLK_TC_FUA	= 1 << 2,	/* fua requests */
>> 
>> I would prefer to replace BARRIER with FLUSH, as I think they are closer
>> relatives.  Doing it the way you've suggested would mean that older
>> blktrace user-space would report FUA as a Barrier.
>> 
>
> I thought about that too. But as I said in the changelog, it led to a
> negative number at the rhs of MASC_TC_BIT calculation, so the end result
> was not good.

Yeah, I ran into that when trying this myself.

> In the meantime, I found that Matthew Wilcox posted a patch which
> relocates some REQ_ flags to appropriate positions.
>
> https://lkml.org/lkml/2011/6/2/324
>
> With the patch, it seems ok to replace BARRIER with FLUSH. However it
> looks like the patch isn't included into the tree yet.

OK, that's good to know.

> BTW, I'm thinking about user-space again. I'm not sure it's ok if older
> blktrace tool reports FLUSH/FUA as BARRIER. Actually I posted a patch
> that treats FLUSH as BARRIER [1], and Jens and others commented we
> should not do that.

> To end that, I could leave BLK_TC_BARRIER as is, and add
> BLK_TC_{FLUSH,FUA} at the end of blktrace_cat. But as we exhause space
> in the 16-bit act_mask, it would require a substantial change.
>
> Any thoughts?

This is a rat hole.  ;-)  Expanding the bit space is not easy, since
there is no versioning in the kernel<->userspace protocol.  The only way
I can think of to accomplish that would be to add a new setup ioctl for
the newer blktrace utility.  Then, for the older utility, we would
simply not report any barriers at all.

Really, though, when I look at blktrace output, and I'm trying to figure
out what's going on (assuming a newer kernel and older blktrace), I'd
rather have write cache flushes reported as barriers than FUA.  FUA
isn't supported by all hardware, and I think the cache flush will be
more commensurate with the hit in performance associated with the
barriers of yore.  If we went this route, then we could just overload
barrier with flush and be done with it.

I'm open to other opinions, though.

Cheers,
Jeff


> [1] https://lkml.org/lkml/2011/5/27/206