Re: Question on block group allocation

[Andreas Dilger <adilger@sun.com>]

On Apr 23, 2009  13:08 -0600, Andreas Dilger wrote:
> This is likely the "uninit_bg" feature that is causing the allocations
> to skip groups which are marked BLOCK_UNINIT.  In some sense the benefit
> of skipping the block bitmap read during e2fsck is probably not at all
> beneficial compared to the cost of the extra seeking during IO.  As the
> filesystem gets more full, the BLOCK_UNIIT flags would be cleared anyways,
> so we might as well just keep the early allocations contiguous.
> 
> A simple change to verify this would be something like the following,
> but it hasn't actually been tested.
> 
> --- ./fs/ext4/mballoc.c.uninit    2009-04-08 19:13:13.000000000 -0600
> +++ ./fs/ext4/mballoc.c 2009-04-23 13:02:22.000000000 -0600
> @@ -1742,10 +1723,6 @@ static int ext4_mb_good_group(struct ext
>  	switch (cr) {
>  	case 0:
>  		BUG_ON(ac->ac_2order == 0);
> -		/* If this group is uninitialized, skip it initially */
> -		desc = ext4_get_group_desc(ac->ac_sb, group, NULL);
> -		if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT))
> -			return 0;
>  
>  		bits = ac->ac_sb->s_blocksize_bits + 1;
>  		for (i = ac->ac_2order; i <= bits; i++)
> @@ -2039,9 +2035,7 @@ repeat:
>  			ac->ac_groups_scanned++;
>  			desc = ext4_get_group_desc(sb, group, NULL);
> -			if (cr == 0 || (desc->bg_flags &
> -				cpu_to_le16(EXT4_BG_BLOCK_UNINIT) &&
> -				ac->ac_2order != 0))
> +			if (cr == 0)
>  				ext4_mb_simple_scan_group(ac, &e4b);
>  			else if (cr == 1 &&
>  					ac->ac_g_ex.fe_len == sbi->s_stripe)

Because this is actually proving to be useful:

Signed-off-by: Andreas Dilger <adilger@sun.com>

As we discussed in the call, I suspect BLOCK_UNINIT was more useful in the
past when directories were spread over all groups evenly (pre-Orlov), and
before flex_bg where seeking to read all of the bitmaps was a slow and
painful process.  For flex_bg it could be WORSE to skip bitmap reads because
instead of doing contiguous 64kB reads it may now doing read 4kB, seek,
read 4kB, seek, etc.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.