Re: [LSF/MM TOPIC] Software RAID Support for NV-DIMM

Dan Williams
2019-02-16T17:05:58Z

On Fri, Feb 15, 2019 at 9:40 PM Dave Chinner <david@fromorbit.com> wrote:
>
> On Sat, Feb 16, 2019 at 04:31:33PM +1100, Dave Chinner wrote:
> > On Fri, Feb 15, 2019 at 10:57:12AM +0100, Johannes Thumshirn wrote:
> > > (This is a joint proposal with Hannes Reinecke)
> > >
> > > Servers with NV-DIMM are slowly emerging in data centers but one key feature
> > > for reliability of these systems hasn't been addressed up to now, data
> > > redundancy.
> > >
> > > While it would be best to solve this issue in the memory controller of the CPU
> > > itself, I don't see this coming in the next few years. This puts us as the OS
> > > in the burden to create the redundant copies of data for the users.
> > >
> > > If we leave of the DAX support Linux' software RAID implementations (MD,
> > > device-mapper and BTRFS RAID) do already work on top of pmem devices, but they
> > > are incompatible with DAX.
> > >
> > > In this session Hannes and I would like to discuss eventual ways how we as an
> > > operating system can mitigate these issues for our users.
> >
> > We've supported this since mid 2018 and commit ba23cba9b3bd ("fs:
> > allow per-device dax status checking for filesystems"). That is,
> > we can have DAX on the XFS RT device indepently of the data device.
> >
> > That is, you set up pmem in three segments - two small identical
> > segments start get mirrored with RAID1 as the data device, and
> > the remainder as a block device that is dax capable set up as the
> > XFS realtime device. Set the RTINHERIT bit on the root directory at
> > mkfs time ("-d rtinherit=1") and then all the data goes to the DAX
> > capable realtime device, and all the metadata goes to the software
> > raided pmem block devices that aren't DAX capable.
> >
> > Problem already solved, yes?
>
> Sorry, this was meant to be a reply to Dan's email commenting about
> some people needing mirrored metadata, not the parent that was
> talking about whole device RAID...
>
> i.e. mirrored metadata w/ FS-DAX for data should already be a solved
> problem...

Ah true, thanks for the clarification. I'll give it a try, the last
time I looked RT configurations failed with DAX, but perhaps that's
been fixed and I can drop if from my list of broken DAX items.