linux-ext4 - Re: [PATCH] avoid scanning bitmaps for group preallocation

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Date:	Fri, 26 Mar 2010 11:58:49 -0600
From:	Andreas Dilger <adilger@....com>
To:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] avoid scanning bitmaps for group preallocation

On 2010-03-26, at 04:28, Aneesh Kumar K. V wrote:
> On Mon, 22 Mar 2010 16:03:10 -0600, Andreas Dilger <adilger@....com>  
> wrote:
>> Here is the patch I mentioned today on the call.  It avoids (or at
>> least reduces) serious latency (10 minutes or more) on a large
>> filesystem (8TB+) on the first write, if the filesystem is nearly
>> full.  The latency is entirely due to seeking to read the block
>> bitmaps, so is considerably less serious on flex_bg formatted
>> filesystems.
>
> @@ -2029,9 +2028,12 @@ repeat:
> +		/* If there's no chance that this group has a better
> +		 * extent, just skip it instead of seeking to read
> +		 * block bitmap from disk. Initially ac_b_ex.fe_len = 0,
> +		 * so this always skips groups with no free space. */
>  		grp = ext4_get_group_info(sb, group);
> -		if (grp->bb_free == 0)
> +		if (grp->bb_free <= ac->ac_b_ex.fe_len)
>  			continue;
>
> I was wondering whether we need to make sure we also use criteria  
> value
> when checking for bb_free. If we are really low on space we may want  
> to
> return what is left right ?. Or does ac_b_ex take care of that ?


ac_b_ex is the best currently ALLOCATED extent, so mballoc wouldn't  
ever select an extent that is smaller than ac_b_ex.fe_len.  That means  
it is pointless to even look at a group which has fewer free blocks  
than ac_b_ex.fe_len.

Later, after the group information is loaded, ldiskfs_mb_good_group()  
will skip the group if the average fragment size is smaller than the  
GOAL extent, but only for certain criterion levels.  At the highest  
criterion, any group with free blocks will be scanned.

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

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