linux-kernel - Re: [PATCH 14/15] libnvdimm: support read-only btt backing devices

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Message-ID: <20150623193043.GE1971@linux.intel.com>
Date:	Tue, 23 Jun 2015 15:30:43 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	Jens Axboe <axboe@...nel.dk>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 14/15] libnvdimm: support read-only btt backing devices

On Mon, Jun 22, 2015 at 08:30:28AM +0200, Christoph Hellwig wrote:
> > Good to hear that we don't need BTT for XFS v5, can we make the
> > guarantee for all filesystems that may want to support DAX?  I still
> > think stacking is a natural fit for this problem.
> 
> I can't make any guarantees, especially not without verification.  But
> if correctly implemented any filesystems that does out of place metadata
> writes (and that includes a traditional log) and uses checksum to ensure
> the integrity of these updates it should be fine.  You'd still have
> the issue of sector atomicy of file I/O though.

Is ext4 one of the filesystems that copes with torn updates to the log?
I see there's a checksum in the tail of at least some blocks, but I'd
like someone who understands ext4 to reassure me that it also doesn't
need the ability to put its log on a BTT.
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