linux-ext4 - Re: [PATCH] mballoc: allocate stripe-multiple IOs on stripe boundaries

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Date:	Wed, 14 Jul 2010 15:33:30 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>,
	Mike Snitzer <snitzer@...hat.com>
Subject: Re: [PATCH] mballoc: allocate stripe-multiple IOs on stripe boundaries

Ah, good catch.  We don't notice this on Lustre, since we always use at most 1MB writes from the network and always configure with 1MB stripe size.

On 2010-07-14, at 15:10, Eric Sandeen wrote:
> For some reason, today mballoc only allocates IOs which are exactly
> stripe-sized on a stripe boundary.  If you have a multiple (say, a
> 128k IO on a 64k stripe) you may end up unaligned.
> 
> It seems to me that a simple change to align stripe-multiple IOs
> on stripe boundaries would be a very good idea, unless this breaks
> some other mballoc heuristic for some reason...
> 
> Reported-by: Mike Snitzer <snitzer@...hat.com>
> Signed-off-by: Eric Sandeen <sandeen@...hat.com>
> ---
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 12b3bc0..f64a439 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -1821,8 +1821,7 @@ void ext4_mb_complex_scan_group(struct ext4_allocation_context *ac,
> 
> /*
> * This is a special case for storages like raid5
> - * we try to find stripe-aligned chunks for stripe-size requests
> - * XXX should do so at least for multiples of stripe size as well
> + * we try to find stripe-aligned chunks for stripe-size-multiple requests
> */
> static noinline_for_stack
> void ext4_mb_scan_aligned(struct ext4_allocation_context *ac,
> @@ -2094,8 +2093,8 @@ repeat:
> 			ac->ac_groups_scanned++;
> 			if (cr == 0)
> 				ext4_mb_simple_scan_group(ac, &e4b);
> -			else if (cr == 1 &&
> -					ac->ac_g_ex.fe_len == sbi->s_stripe)
> +			else if (cr == 1 && sbi->s_stripe &&
> +					!(ac->ac_g_ex.fe_len % sbi->s_stripe))
> 				ext4_mb_scan_aligned(ac, &e4b);
> 			else
> 				ext4_mb_complex_scan_group(ac, &e4b);
> 
> 
> --
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Cheers, Andreas





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