Re: libata FUA revisited

From: Ric Wheeler
Date: Thu Feb 22 2007 - 17:43:47 EST

Tejun Heo wrote:
[cc'ing Ric, Hannes and Dongjun, Hello. Feel free to drag other people in.]

Robert Hancock wrote:
Jens Axboe wrote:
But we can't really change that, since you need the cache flushed before
issuing the FUA write. I've been advocating for an ordered bit for
years, so that we could just do:


normal operation -> barrier issued -> write barrier FUA+ORDERED
-> normal operation resumes

So we don't have to serialize everything both at the block and device
level. I would have made FUA imply this already, but apparently it's not
what MS wanted FUA for, so... The current implementations take the FUA
bit (or WRITE FUA) as a hint to boost it to head of queue, so you are
almost certainly going to jump ahead of already queued writes. Which we
of course really do not.

Yeah, I think if we have tagged write command and flush tagged (or
barrier tagged) things can be pretty efficient. Again, I'm much more
comfortable with separate opcodes for those rather than bits changing
the behavior.

Another idea Dongjun talked about while drinking in LSF was ranged
flush. Not as flexible/efficient as the previous option but much less
intrusive and should help quite a bit, I think.

I think that FUA was designed for a different use case than what Linux
is using barriers for currently. The advantage with FUA is when you have
"before barrier", "after barrier" and "don't care" sets, where only the
specific things you care about ordering are in the before/after barrier
sets. Then you can do this:

Issue all before barrier requests with FUA bit set
Wait for all those to complete
Issue all after barrier requests with FUA bit set
Wait for all those to complete

A couple of issues with this would be in how to support our current semantics of fsync(). Today, the flush behavior of the barrier/fsync combination means that applications can have a hard promise of data on platter for any file after a successful fsync command.

If I understand correctly, to get a similar semantic from a pure FUA implementation would require us to tag all file IO as FUA.

I suspect that this would actually be less efficient since it would not allow the drives to reorder IO's up to the point that we actually care (fsync time).

The other big user of barriers is the internal transaction of journaled file systems. It would seem that we would need to tag each write from the journal with a FUA IO as well. Again, we might actually go more slowly in some cases as you mention below.

The limited queue depth of NCQ would seem to make it much harder to have a win in this case...

Meanwhile a bunch of "don't care" requests could be going through on the
device in the background. If we could do this, then I think there would
be an advantage. Right now, it just saves a command to the drive when
we're flushing on the post-barrier writes.

This would only be efficient with NCQ FUA, because regular FUA forces
the requests to complete serially, whereas in this case we don't really
care what order the individual requests finish, we just care about the
ordering of the pre vs. post barrier requests.

Yeap, that makes sense too but that possibly requires intrusive changes
in fs layer and limited NCQ queue depth might become a bottleneck too.

I'm not too nervous about the FUA write commands, I hope we can safely
assume that if you set the FUA supported bit in the id AND the write fua
command doesn't get aborted, that FUA must work. Anything else would
just be an immensely stupid implementation. NCQ+FUA is more tricky, I
agree that it being just a command bit does make it more likely that it
could be ignored. And that is indeed a danger. Given state of NCQ in
early firmware drives, I would not at all be surprised if the drive
vendors screwed that up too.

Yeap, I bet someone did. :-)

But, since we don't have the ordered bit for NCQ/FUA anyway, we do need
to drain the drive queue before issuing the WRITE/FUA. And at that point
we may as well not use the NCQ command, just go for the regular non-NCQ
FUA write. I think that should be safe.


Aside from the issue above, as I mentioned elsewhere, lots of NCQ drives
don't support non-NCQ FUA writes..

To me, using the NCQ FUA bit on such drives doesn't seem to be a good
idea. Maybe I'm just too chicken but it's not like we can gain a lot
from doing FUA at this point. Are there a lot of drives which support
NCQ but not FUA opcodes?


Anything new (firmware included) is likely to be shaky on initial deployment. Caution is certainly the way to go on this ;-)


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at