linux-ext4 - Re: [PATCH 2/2] Add batched discard support for ext4.

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Date:	Mon, 26 Apr 2010 11:27:12 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Mark Lord <kernel@...savvy.com>, Ric Wheeler <rwheeler@...hat.com>,
	Eric Sandeen <sandeen@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>,
	Jeff Moyer <jmoyer@...hat.com>, linux-ext4@...r.kernel.org,
	Edward Shishkin <eshishki@...hat.com>,
	Eric Sandeen <esandeen@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 2/2] Add batched discard support for ext4.

On Mon, Apr 26, 2010 at 10:42 AM, Martin K. Petersen
<martin.petersen@...cle.com> wrote:
>>>>>> "Mark" == Mark Lord <kernel@...savvy.com> writes:
>
> Mark> So.. again.. in an ideal kernel, I'd like to see use of larger
> Mark> ranges (and _multiple_ ranges) per TRIM command.  And options for
> Mark> the kernel to do it automatically (-o discard), as well as an
> Mark> ioctl() interface for userspace to "scrub" (or "wipe") all free
> Mark> ranges in a gradual fashion.
>
> Again: Discard splitting, merging, and coalescing is coming.  I'm
> working on it.  It's not exactly trivial.
>
> My objection was purely wrt. nuking realtime discard in favor of
> scrubbing.  We absolutely need both approaches.
>
> But I'm not going to let crappy SSD firmware implementations set the
> direction for what I'm working on.  It is much more interesting to
> ensure that we work well for the devices that'll be shipping 6-12 months
> from now (at the time where this stuff will realistically end up in
> distro kernels).
>
> --
> Martin K. Petersen      Oracle Linux Engineering
>

Is this an agreed summary as relates to ext4:

1) Current "-o discard" causes real-time calls to discard.  Although
not optimized for current generation hardware, both block-layer
optimizations and new hardware are coming, so it needs to be kept.

2) Kernel based discard scrubbing - Doesn't currently exist in 2.6.34,
all agree that for the LVM, mdraid, btrfs cases it is needed and there
is no linux tool (kernel or userspace) at present.  The Lukas proposed
patch is userspace invoked so a mount option is not needed.

3) Userspace discard is available today and works well with ext4 on a
single JBOD SSD which will be the typical laptop use as one example.

Mark, or anyone, do you think it would be a bad idea for me to push
for Opensuse 11.3 (2.6.34 based) to use wiper.sh as a standard ext4
discard tool?  hdparm and wiper.sh are already in the distro, it just
needs a cron entry and maybe some basic management infrastructure.
They're at feature freeze for 11.3, so I don't know if I can get it in
or not.

If and when the suse team want to move to the kernel based scrubber,
they can.  ie. a year from now when the next release after 11.3 comes
out.

Greg
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