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

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Date:	Wed, 14 Jul 2010 13:43:30 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
cc:	Lukas Czerner <lczerner@...hat.com>, eshishki@...hat.com,
	jmoyer@...hat.com, rwheeler@...hat.com, linux-ext4@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: [PATCH 2/2] Add batched discard support for ext4

On Wed, 14 Jul 2010, Dmitry Monakhov wrote:

> Lukas Czerner <lczerner@...hat.com> writes:
> 
> > On Wed, 14 Jul 2010, Dmitry Monakhov wrote:
> >
> >> Lukas Czerner <lczerner@...hat.com> writes:
> >> 
> >> > Walk through each allocation group and trim all free extents. It can be
> >> > invoked through TRIM ioctl on the file system. The main idea is to
> >> > provide a way to trim the whole file system if needed, since some SSD's
> >> > may suffer from performance loss after the whole device was filled (it
> >> > does not mean that fs is full!).
> >> >
> >> > It search fro free extents in each allocation group. When the free
> >> > extent is found, blocks are marked as used and then trimmed. Afterwards
> >> > these blocks are marked as free in per-group bitmap.
> >> Looks ok, except two small notes:
> >> 1) trim_fs is a time consuming operation and we have to add
> >>    condresced, and signal_pending checks to allow user to interrupt
> >>    cmd if necessery. See patch attached.
> >
> > Hi, Dimitry
> >
> > thanks for your patch! Although I have one question:
> >
> >
> >  	for (group = 0; group < ngroups; group++) {
> > -		int err;
> > -
> > -		err = ext4_mb_load_buddy(sb, group, &e4b);
> > -		if (err) {
> > +		ret = ext4_mb_load_buddy(sb, group, &e4b);
> > +		if (ret) {
> >  			ext4_error(sb, "Error in loading buddy "
> >  					"information for %u", group);
> > -			continue;
> > +			break;
> >  		}
> >
> > Is there really need to jump out from the loop and exit in the case of
> > load_buddy failure ? Next group may very well succeed in loading buddy,
> > or am I missing something ?
> Well, it may fail due to -ENOMEM which is not scary but in some places
> it may fail due to EIO which is a very bad sign. So i think it is
> slightly dangerous to continue if we have found a same group.

Ok, it seems reasonable.

> >
> >> 2) IMHO runtime trim support is useful sometimes, for example when
> >>    user really care about data security i.e. unlinked file should be
> >>    trimmed ASAP. I think we have to provide 'secdel' mount option
> >>    similar to secdeletion flag for inode, but this is another story
> >>    not directly connected with the patch.
> >
> > I like the idea, but IMO this should work for any underlying storage,
> > not just for SSDs.
> Off course. We may use blkdev_issue_zeroout() if disk does not support
> discard with zeroing.
> >

-Lukas
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