[cc'ing Jeff, Alan, Mark and Jens. Hi!]
Hello, Robert.
Robert Hancock wrote:
Well, we should be able to determine that experimentally (at least on
specific controllers) with a little test program that just writes little
bits of data and fsyncs repeatedly (assuming that does in fact trigger
FUAs currently..) If it runs faster than the drive could possibly be
rewriting the physical disk then obviously the FUA bit is not getting
through and/or not respected and we can blacklist FUA on that controller.
That's right.
Also, the FUA bit in the NCQ commands is in the device register, so it's
not like the PMP fields where it's not used for anything else and so the
controller messing with it wouldn't be otherwise noticed..
Yeap, I just wanted to point out (so the FWIW) that seemingly innocent
ahci does mangle with some part of the FIS given in the memory. I agree
that this is much unlikely with the FUA bit.
So, actually, I was thinking about *always* using the non-NCQ FUA
opcode. As currently implemented, FUA request is always issued by
itself, so NCQ doesn't make any difference there. So, I think it
would be better to turn on FUA on driver-by-driver basis whether the
controller supports NCQ or not.
Unfortunately not all drives that support NCQ support the non-NCQ FUA
commands (my Seagates are like this).
And I'm a bit scared to set FUA bit on such drives and trust that it
will actually do FUA, so our opinions aren't too far away from each
other. :-)
There's definitely a potential advantage to FUA with NCQ - if you have
non-synchronous accesses going on concurrently with synchronous ones, if
you have to use non-NCQ FUA or flush cache commands, you have to wait
for all the IOs of both types to drain out before you can issue the
flush (since those can't be overlapped with the NCQ read/writes). And if
you can only use flush cache, then you're forcing all the writes to be
flushed including the non-synchronous ones you didn't care about.
Whether or not the block layer currently exploits this I don't know, but
it definitely could.
The current barrier implementation uses the following sequences for
no-FUA and FUA cases.
1. w/o FUA
normal operation -> barrier issued -> drain IO -> flush -> barrier
written -> flush -> normal operation resumes
2. w/ FUA
normal operation -> barrier issued -> drain IO -> flush -> barrier
written / FUA -> normal operation resumes
So, the FUA write is issued by itself. This isn't really efficient and
frequent barriers impact the performance badly. If we can change that
NCQ FUA will be certainly beneficial.
Well, I might be being too paranoid but silent FUA failure would be
really hard to diagnose if that ever happens (and I'm fairly certain
that it will on some firmwares).
Well, there are also probably drives that ignore flush cache commands or
fail to do other things that they should. There's only so far we can go
in coping if the firmware authors are being retarded. If any drive is
broken like that we should likely just blacklist NCQ on it entirely as
obviously little thought or testing went into the implementation..
FLUSH has been around quite long time now and most drives don't have
problem with that. FUA on ATA is still quite new and libata will be the
first major user of it if we enable it by default. It just seems too
easy to ignore that bit and successfully complete the write - there
isn't any safety net as opposed to using a separate opcode. So, I'm a
bit nervous.
Any comments, people?
Thanks.
--
tejun
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