Re: Modern uses of CONFIG_XFS_RT

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

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.

> > 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?

> 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.

--D

> [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
> 
>   Luis