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

From: Dave Chinner <david@fromorbit.com>
To: Johannes Thumshirn <jthumshirn@suse.de>
Cc: lsf-pc@lists.linux-foundation.org, linux-block@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-nvdimm@lists.01.org,
	linux-btrfs@vger.kernel.org, hare@suse.de
Subject: Re: [LSF/MM TOPIC] Software RAID Support for NV-DIMM
Date: Sat, 16 Feb 2019 16:39:57 +1100	[thread overview]
Message-ID: <20190216053957.GU20493@dastard> (raw)
In-Reply-To: <20190216053133.GT20493@dastard>

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...

Cheers,

Dave.
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
> 

-- 
Dave Chinner
david@fromorbit.com