From: Eric Sandeen <sandeen@redhat.com>
Subject: Re: [PATCH] ext4: Change the handling of RAID stripe width
Date: Wed, 06 Jul 2011 16:18:40 -0500
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Cc: Andreas Dilger <adilger.kernel@dilger.ca>,
	linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Theodore Ts'o" <tytso@mit.edu>
To: Dan Ehrenberg <dehrenberg@google.com>
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On 7/6/11 3:47 PM, Dan Ehrenberg wrote:
> Previously, the stripe width was blindly used for determining the size
> of allocations. Now, the stripe width is used as a hint for the initial
> mb_group_prealloc; if it is greater than 1, then we make sure that
> mb_group_prealloc is some multiple of it, and otherwise it is ignored.
> mb_group_prealloc is always usable to adjust the preallocation strategy,
> not just when the stripe-width is 0 as before.
> 
> Signed-off-by: Dan Ehrenberg <dehrenberg@google.com>
> ---
>  fs/ext4/mballoc.c |   40 +++++++++++++++++++++++++++++-----------
>  1 files changed, 29 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 6ed859d..710c27f 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -127,13 +127,14 @@
>   * based on file size. This can be found in ext4_mb_normalize_request. If
>   * we are doing a group prealloc we try to normalize the request to
>   * sbi->s_mb_group_prealloc. Default value of s_mb_group_prealloc is
> - * 512 blocks. This can be tuned via
> - * /sys/fs/ext4/<partition/mb_group_prealloc. The value is represented in
> - * terms of number of blocks. If we have mounted the file system with -O
> + * 512 blocks. If we have mounted the file system with -O
>   * stripe=<value> option the group prealloc request is normalized to the
> - * stripe value (sbi->s_stripe)
> + * the smallest multiple of the stripe value (sbi->s_stripe) which is
> + * greater than the default mb_group_prealloc. This can be tuned via
> + * /sys/fs/ext4/<partition>/mb_group_prealloc. The value is represented in
> + * terms of number of blocks.
>   *
> - * The regular allocator(using the buddy cache) supports few tunables.
> + * The regular allocator (using the buddy cache) supports a few tunables.
>   *
>   * /sys/fs/ext4/<partition>/mb_min_to_scan
>   * /sys/fs/ext4/<partition>/mb_max_to_scan
> @@ -2471,7 +2472,26 @@ int ext4_mb_init(struct super_block *sb, int needs_recovery)
>  	sbi->s_mb_stats = MB_DEFAULT_STATS;
>  	sbi->s_mb_stream_request = MB_DEFAULT_STREAM_THRESHOLD;
>  	sbi->s_mb_order2_reqs = MB_DEFAULT_ORDER2_REQS;
> +	/*
> +	 * If the stripe width is 1, this makes no sense and
> +	 * we set it to 0 to turn off stripe handling code.
> +	 */
> +	if (sbi->s_stripe == 1)
> +		sbi->s_stripe = 0;

This strikes me as a weird band-aid-y place to fix this up.

Wouldn't it be better suited for the option-parsing code, and/or
in ext4_get_stripe_size()?  Why let a value of 1 get this far
only to override it here?

-Eric

>  	sbi->s_mb_group_prealloc = MB_DEFAULT_GROUP_PREALLOC;
> +	/*
> +	 * If there is a s_stripe > 1, then we set the s_mb_group_prealloc
> +	 * to the lowest multiple of s_stripe which is bigger than
> +	 * the s_mb_group_prealloc as determined above. We want
> +	 * the preallocation size to be an exact multiple of the
> +	 * RAID stripe size so that preallocations don't fragment
> +	 * the stripes.
> +	 */
> +	if (sbi->s_stripe > 1) {
> +		sbi->s_mb_group_prealloc = roundup(
> +			sbi->s_mb_group_prealloc, sbi->s_stripe);
> +	}
>  
>  	sbi->s_locality_groups = alloc_percpu(struct ext4_locality_group);
>  	if (sbi->s_locality_groups == NULL) {
> @@ -2830,8 +2850,9 @@ out_err:
>  
>  /*
>   * here we normalize request for locality group
> - * Group request are normalized to s_strip size if we set the same via mount
> - * option. If not we set it to s_mb_group_prealloc which can be configured via
> + * Group request are normalized to s_mb_group_prealloc, which goes to
> + * s_strip if we set the same via mount option.
> + * s_mb_group_prealloc can be configured via
>   * /sys/fs/ext4/<partition>/mb_group_prealloc
>   *
>   * XXX: should we try to preallocate more than the group has now?
> @@ -2842,10 +2863,7 @@ static void ext4_mb_normalize_group_request(struct ext4_allocation_context *ac)
>  	struct ext4_locality_group *lg = ac->ac_lg;
>  
>  	BUG_ON(lg == NULL);
> -	if (EXT4_SB(sb)->s_stripe)
> -		ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_stripe;
> -	else
> -		ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc;
> +	ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc;
>  	mb_debug(1, "#%u: goal %u blocks for locality group\n",
>  		current->pid, ac->ac_g_ex.fe_len);
>  }