Re: Modern uses of CONFIG_XFS_RT

Eric Sandeen
2020-02-20T02:12:27Z

On 2/19/20 6:17 PM, Luis Chamberlain wrote:
> On Wed, Feb 19, 2020 at 02:01:04PM -0800, Darrick J. Wong wrote:
>> On Wed, Feb 19, 2020 at 05:55:02PM +0000, Luis Chamberlain wrote:
>>> On Wed, Feb 19, 2020 at 09:09:45AM -0800, Darrick J. Wong wrote:
>>>> On Wed, Feb 19, 2020 at 02:38:24PM +0000, Luis Chamberlain wrote:
>>>>> On Wed, Feb 19, 2020 at 03:32:27PM +0100, Carlos Maiolino wrote:
>>>>>> On Wed, Feb 19, 2020 at 01:57:15PM +0000, Luis Chamberlain wrote:
>>>>>>> I hear some folks still use CONFIG_XFS_RT, I was curious what was the
>>>>>>> actual modern typical use case for it. I thought this was somewhat
>>>>>>> realted to DAX use but upon a quick code inspection I see direct
>>>>>>> realtionship.
>>>>>>
>>>>>> Hm, not sure if there is any other use other than it's original purpose of
>>>>>> reducing latency jitters. Also XFS_RT dates way back from the day DAX was even a
>>>>>> thing. But anyway, I don't have much experience using XFS_RT by myself, and I
>>>>>> probably raised more questions than answers to yours :P
>>>>>
>>>>> What about another question, this would certainly drive the users out of
>>>>> the corners: can we remove it upstream?
>>>>
>>>> My DVR and TV still use it to record video data.
>>>
>>> Is anyone productizing on that though?
>>>
>>> I was curious since most distros are disabling CONFIG_XFS_RT so I was
>>> curious who was actually testing this stuff or caring about it.
>>
>> Most != All. We enabled it here, for development of future products.
>
> Ah great to know, thanks!
>
>>>> I've also been pushing the realtime volume for persistent memory devices
>>>> because you can guarantee that all the expensive pmem gets used for data
>>>> storage, that the extents will always be perfectly aligned to large page
>>>> sizes, and that fs metadata will never defeat that alignment guarantee.
>>>
>>> For those that *are* using XFS in production with realtime volume with dax...
>>> I wonder whatcha doing about all these tests on fstests which we don't
>>> have a proper way to know if the test succeeded / failed [0] when an
>>> external logdev is used, this then applies to regular external log dev
>>> users as well [1].
>>
>> Huh? How did we jump from realtime devices to external log files?
>
> They share the same problem with fstests when using an alternative log
> device, which I pointed out on [0] and [1].
>
> [0] https://github.com/mcgrof/oscheck/blob/master/expunges/linux-next-xfs/xfs/unassigned/xfs_realtimedev.txt
> [1] https://github.com/mcgrof/oscheck/blob/master/expunges/linux-next-xfs/xfs/unassigned/xfs_logdev.txt
>
>>> Which makes me also wonder then, what are the typical big users of the
>>> regular external log device?
>>>
>>> Reviewing a way to address this on fstests has been on my TODO for
>>> a while, but it begs the question of how much do we really care first.
>>> And that's what I was really trying to figure out.
>>>
>>> Can / should we phase out external logdev / realtime dev? Who really is
>>> caring about this code these days?
>>
>> Not many, I guess. :/
>>
>> There seem to be a lot more tests these days that use dmflakey on the
>> data device to simulate a temporary disk failure... but those aren't
>> going to work for external log devices because they seem to assume that
>> what we call the data device is also the log device.
>
> That goes to show that the fstests assumption on a shared data/log device was
> not only a thing of the past, its still present, and unless we address
> soon, the gap will only get bigger.
>
> OK thanks for the feedback. The situation in terms of testing rtdev or
> external logs seems actually worse than I expected given the outlook for
> the future and no one seeming to really care too much right now. If the
> dax folks didn't care, then the code will likely just bit rot even more.
> Is it too nutty for us to consider removing it as a future goal?

Less nutty would be to analyze the failures and fix the tests.

Here's a start, I'll send this one to fstests.

diff --git a/common/repair b/common/repair
index 5a9097f4..cf69dde9 100644
--- a/common/repair
+++ b/common/repair
@@ -9,8 +9,12 @@ _zero_position()
value=$1
struct="$2"

+ SCRATCH_OPTIONS=""
+ [ "$USE_EXTERNAL" = yes -a ! -z "$SCRATCH_LOGDEV" ] && \
+ SCRATCH_OPTIONS="-l$SCRATCH_LOGDEV"
+
# set values for off/len variables provided by db
- eval `xfs_db -r -c "$struct" -c stack $SCRATCH_DEV | perl -ne '
+ eval `xfs_db -r -c "$struct" -c stack $SCRATCH_OPTIONS $SCRATCH_DEV | perl -ne '
if (/byte offset (\d+), length (\d+)/) {
print "offset=$1\nlength=$2\n"; exit
}'`
diff --git a/tests/xfs/030 b/tests/xfs/030
index efdb6a18..e1cc32ef 100755
--- a/tests/xfs/030
+++ b/tests/xfs/030
@@ -77,7 +77,10 @@ else
_scratch_unmount
fi
clear=""
-eval `xfs_db -r -c "sb 1" -c stack $SCRATCH_DEV | perl -ne '
+SCRATCH_OPTIONS=""
+[ "$USE_EXTERNAL" = yes -a ! -z "$SCRATCH_LOGDEV" ] && \
+ SCRATCH_OPTIONS="-l$SCRATCH_LOGDEV"
+eval `xfs_db -r -c "sb 1" -c stack $SCRATCH_OPTIONS $SCRATCH_DEV | perl -ne '
if (/byte offset (\d+), length (\d+)/) {
print "clear=", $1 / 512, "\n"; exit
}'`