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Date:	Thu, 10 Jan 2008 09:58:59 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	cmm@...ibm.com, tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Use superblock s_raid_stripe_width as stripe
	size during block allocation.

On Wed, Jan 09, 2008 at 04:36:27PM -0700, Andreas Dilger wrote:
> On Jan 09, 2008  22:37 +0530, Aneesh Kumar K.V wrote:
> > The stipe size used during block allocation is calculated as below.
> > a) if we specify a stripe=<value> option using mount time. Use that value.
> > b) if not use the value specified in super block.
> > b) If the value specfied at mount time is greater than blocks per group use
> > the value specified ini the super block.
> 
> What if the value on disk is also bad?  I'd add (after the second 'b' :-):
> 
> d) if s_stripe is still > s_blocks_per_group try s_raid_stride
> e) if s_stripe is still > s_blocks_per_group use 0

But i guess mke2fs and tune2fs should validate the value of
s_raid_stripe_width and s_raid_stride. Both of them should be less that
blocks per group. Should I add extra check in the kernel for them ?

> 
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> > ---
> >  fs/ext4/super.c |   10 ++++++++++
> >  1 files changed, 10 insertions(+), 0 deletions(-)
> > 
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index db1edc8..10330eb 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -2136,6 +2136,16 @@ static int ext4_fill_super (struct super_block *sb, void *data, int silent)
> >  	sbi->s_rsv_window_head.rsv_alloc_hit = 0;
> >  	sbi->s_rsv_window_head.rsv_goal_size = 0;
> >  	ext4_rsv_window_add(sb, &sbi->s_rsv_window_head);
> > +	/*
> > +	 * set the stripe size. If we have specified it via mount option, then
> > +	 * use the mount option value. If the value specified at mount time is
> > +	 * greater than the blocks per group use the super block value.
> > +	 * Allocator needs it be less than blocks per group.
> > +	 */
> > +	if (!sbi->s_stripe ||
> > +			sbi->s_stripe >= sbi->s_blocks_per_group) {
> 

So what do you think should it be > or >=. Looking at the mballoc I
guess it should work with stripe size equal to blocks per group. I am
not sure how efficient the allocation would be though.


-aneesh
-
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