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

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Date:	Mon, 22 Jun 2015 09:59:58 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Jens Axboe <axboe@...nel.dk>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Boaz Harrosh <boaz@...xistor.com>,
	"Kani, Toshimitsu" <toshi.kani@...com>,
	"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 9:57 AM, Christoph Hellwig <hch@....de> wrote:
> On Mon, Jun 22, 2015 at 09:54:51AM -0700, Dan Williams wrote:
>> > I don't see why you're comparing with MD and DM here.  MD and DM
>> > sit cleanly ontop of any block device.  If btt was independent of
>> > libnvdimm and just used ->rw_bytes we could see it as this.
>> >
>> > But it's all a giant entangled mess, where btt for example is probed
>> > by libnvdimm.  At the same time pmem.c isn't really a true block
>> > driver, it's really just a trivial shim between the block API
>> > and pmem-style memcpy.  Especially with the proper pmem API btt
>> > would become cleaner just calling that directly.
>>
>> The pmem api does nothing to fix torn sectors, there's no extra
>> atomicity guarantees that come from those instructions.
>
> Of course not.  And neither does pmem.c help with you in any way.
>
> That's the point:  btt should be a peer to pmem.c, not on top of it
> as there's no value add in pmem.c for it, and they are logically peers.
>
>> Well, let's start with per-disk btt and see where that gets us, we can
>> always ramp up complexity later.  I'd just as soon make the default
>> opt-in/out a Kconfig toggle with a sysfs override.
>
> Kconfig or sysfs are both utterly horrible choices.  It's a disk format
> choice so it needs to be persisted.

Of course it will be persisted with an on disk BTT superblock.
Establishing that superblock by default and deleting at on-demand are
via Kconfig and sysfs.
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