Re: [PATCH 1/4] xfs: add online discard support

[Alex Elder <aelder@sgi.com>]

On Fri, 2011-05-20 at 13:43 +0200, Lukas Czerner wrote:
> On Fri, 20 May 2011, Christoph Hellwig wrote:
> 
> > On Thu, May 19, 2011 at 04:53:44PM -0500, Alex Elder wrote:
> > > The first is, why not support it for non-delaylog?
> > 
> > Because:
> > 
. . .
> > 
> > It's a bad idea to do the sort twice for no good reason, and adding
> > another parameter to further overload xfs_alloc_busy_clear behaviour
> > doesn't seem smart either.
> > 
> > > 		if (error == EOPNOTSUPP) {
> > > 			/*
> > > 			 * Report this once per mount point somehow?
> Actually, this is a good idea see https://lkml.org/lkml/2011/5/5/162. So
> you will get EOPNOTSUPP *only* if the device (as a whole) does not
> support discard.

I.e., if *any* component of the underlying storage supports
discard, the aggregate device supports discard.  (Rather than
only if *all* components support it.)  This seems pretty reasonable.

> > > 			 * If so, turn off the mount option?
> Not so good idea, as some people mentioned several times, you can change
> the devices in dmsetup to SSD (for example) without umount and you would
> like your previous mount option to work. In the opposite case, the user
> just gets warning (once a day perhaps?) and its up to him to deal with
> it.

Sorry, I wasn't following that discussion closely.

> Or, we can turn it of (with warning) and rely on the user to notice that
> it is turned off. But I would rather not rely on that.

I agree with you.  I didn't realize the underlying storage could
change attributes without notification of some kind.  The FS layer
might benefit from knowing when such changes take place.

					-Alex

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