Re: Modern uses of CONFIG_XFS_RT

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Feb 19, 2020 at 08:12:23PM -0600, Eric Sandeen wrote:
> 
> 
> 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 '

_scratch_xfs_db

--D

>  	if (/byte offset (\d+), length (\d+)/) {
>  		print "clear=", $1 / 512, "\n"; exit
>  	}'`
> 
> 
>