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

To: Lukas Czerner <lczerner@xxxxxxxxxx>
Subject: Re: [PATCH 1/4] xfs: add online discard support
From: Alex Elder <aelder@xxxxxxx>
Date: Fri, 20 May 2011 08:57:17 -0500
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, <xfs@xxxxxxxxxxx>
In-reply-to: <alpine.LFD.2.00.1105201330360.5226@xxxxxxxxxxxxxxxxxxxxxxxxxx>
References: <20110504185513.136746538@xxxxxxxxxxxxxxxxxxxxxx> <20110504190011.156999943@xxxxxxxxxxxxxxxxxxxxxx> <1305842024.2825.86.camel@doink> <20110520102430.GA18199@xxxxxxxxxxxxx> <alpine.LFD.2.00.1105201330360.5226@xxxxxxxxxxxxxxxxxxxxxxxxxx>
Reply-to: <aelder@xxxxxxx>
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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