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

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Message-ID: <20150624121152.GD17542@lst.de>
Date:	Wed, 24 Jun 2015 14:11:52 +0200
From:	Christoph Hellwig <hch@....de>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Christoph Hellwig <hch@....de>,
	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 Tue, Jun 23, 2015 at 03:30:43PM -0400, Matthew Wilcox wrote:
> > 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.

In theory it should if the log checksums are enabled, but I wouldn't rely
on it without without confirmation and validation from the ext4 folks.
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