linux-ext4 - Re: [PATCH 2/2] Add batched discard support for ext3

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Date:	Mon, 12 Jul 2010 17:58:46 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Jan Kara <jack@...e.cz>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	jmoyer@...hat.com, rwheeler@...hat.com, eshishki@...hat.com,
	sandeen@...hat.com
Subject: Re: [PATCH 2/2] Add batched discard support for ext3

On Mon, 12 Jul 2010, Jan Kara wrote:

> > 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 for 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.
> > 
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > ---
> >  fs/ext3/balloc.c        |  145 +++++++++++++++++++++++++++++++++++++++++++++++
> >  fs/ext3/super.c         |    1 +
> >  include/linux/ext3_fs.h |    1 +
> >  3 files changed, 147 insertions(+), 0 deletions(-)
> > 
> > diff --git a/fs/ext3/balloc.c b/fs/ext3/balloc.c
> > index a177122..bcee525 100644
> > --- a/fs/ext3/balloc.c
> > +++ b/fs/ext3/balloc.c
> ...
> > +		/**
> > +		 * Allocate contiguous free extents by setting bits in the
> > +		 * block bitmap
> > +		 */
> > +		while (next < max
> > +			&& !ext3_set_bit_atomic(sb_bgl_lock(sbi, group),
> > +					next, bh->b_data)) {
> > +			next++;
> > +		}
>   This is actually wrong. You completely ignore journalling here. You can't
> just go and modify metadata buffer - other process can be modifying it as well
> and writing it to disk and thus your changes will also get written. And if
> a crash happens afterwards before the bitmap is written again, you'll get an
> inconsistent filesystem.
>   Also you have to check whether the block isn't actually still used by a
> running/committing transaction - look at fs/ext3/balloc.c:claim_block() to see
> how you have to allocate free blocks.

I may be wrong, but I thought that since the trim command ensures that
every operation in queue completes before the trim proceed, I do not
need to care much about the journaling and running transaction. But I
will took at it once more..

>   All-in-all won't it be good enough to just freeze the fs, do the trimming,
> and unfreeze it?
> 
> 									Honza
> 

Yes, it can be done that way. But it is king of rough solution.

Thank.
-Lukas
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