Date:	Mon, 2 May 2011 09:16:21 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Alasdair G Kergon <agk@...hat.com>
cc:	Lukas Czerner <lczerner@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>, sandeen@...hat.com,
	Christoph Hellwig <hch@...radead.org>, dm-devel@...hat.com,
	DarkNovaNick@...il.com, linux-lvm@...hat.com,
	linux-ext4@...r.kernel.org
Subject: Re: [dm-devel] do not disable ext4 discards on first discard failure?
 [was: Re: dm snapshot: ignore discards issued to the snapshot-origin
 target]

On Fri, 29 Apr 2011, Alasdair G Kergon wrote:

> On Fri, Apr 29, 2011 at 11:30:47AM +0200, Lukas Czerner wrote:
> > 1. We need to honor all the "discard limits" so the discard bios does
> > not actually fail.
> > 2. If the device is composed of various other devices, we should return
> > -EOPNOTSUPP is none of the devices support discard.
> > 3. We should succeed, if at least one of the devices support discard and
> > it does not fail for any reason.
> > 4. We should not advertise discard_zeroes_data if any of the devices
> > does not zero data or does not support discard.
> > I am not sure how "hard" is to assure those conditions in DM. If those
> > conditions are met, we can rely on consistent information in the layers
> > above.
>  
> Remember that -EOPNOTSUPP return applies only to that one *specific*
> discard.  It does not tell you for sure whether or not another future
> discard to the device will succeed.  (It's a property of offset - if
> there are several devices underneath - and of time - if the device or
> one below it gets reconfigured.)
> 
> The core issue here is whether a filesytem should decide that the
> receipt of a single -EOPNOTSUPP is a reason never to send any more,
> whether a more sophisticated algorithm should be used (considering 
> the proportion/offsets of them over given periods of time and retrying
> later), or whether more comprehensive information about the discard
> capabilities of the device should be presented - and whether this should
> be handled automatically or whether it should be under userspace control
> (i.e. the sysadmin can instruct the filesystem what to do).
> 
> dm's role is simply to handle a discard if it can, and report back
> if it couldn't.  Additionally it could provide more comprehensive
> information about the discard capabilities of the device, but my sense
> is that most consider this unnecessary as normally dm devices will have
> a coherent behaviour throughout.
> 
> Alasdair
> 
> 

Hi Alasdair,

I like the last idea the best. Dm is the only one who has the
overall information about the storage, hence it should provide
some of that information for others to do the right thing.

Christoph mentioned that ext4 is the only one actually disabling
discard on -EOPNOTSUPP, but firstly that's not true (gfs2 and
nilfs does that as well) and secondly it is the right thing to
do! What else  should we be doing when it tells us that "discard is
not supported"? continue trying ? that does not make sense at all!

However when we have dm device where part of the device supports
discard due to underlying hardware capability we just can not return
EOPNOTSUPP from blkdev_issue_discard, because it is just not true! It
does support discard, but it is not consistent through the whole device
and at some layer we should notice that and not let it through. How to
do this is just implementation detail. We can either do it at dm level,
while handling bios, or we can export "do not return eopnotsupp"
information and blkdev_issue_discard() should use that information to do
the right thing.

Thanks!
-Lukas
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