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Date:   Sun, 26 Jan 2020 23:15:49 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Li Dongyang <dongyangli@....com>
Cc:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "adilger@...ger.ca" <adilger@...ger.ca>
Subject: Re: [PATCH v3 4/5] mke2fs: set overhead in super block

On Wed, Nov 20, 2019 at 04:35:27AM +0000, Li Dongyang wrote:
> If overhead is not recorded in the super block, it is caculated
> during mount in kernel, for bigalloc file systems the it takes
> O(groups**2) in time.
> For a 1PB deivce with 32K cluste size it takes ~12 mins to
> mount, with most of the time spent on figuring out overhead.
> 
> While we can not improve the overhead algorithm in kernel
> due to the nature of bigalloc, we can work out the overhead
> during mke2fs and set it in the super block, avoiding calculating
> it every time when it mounts.
> 
> Overhead is s_first_data_block plus internal journal blocks plus
> the block and inode bitmaps, inode table, super block backups and
> group descriptor blocks for every group. This patch introduces
> ext2fs_count_used_clusters(), which calculates the clusters used
> in the block bitmap for the given range.
> 
> When bad blocks are involved, it gets tricky because the blocks
> counted as overhead and the bad blocks can end up in the same
> allocation cluster. In this case we will unmark the bad blocks from
> the block bitmap, convert to cluster bitmap and get the overhead,
> then mark the bad blocks back in the cluster bitmap.
> 
> Reset the overhead to zero when resizing, we can not simplly count
> the used blocks as overhead like we do when mke2fs. The overhead
> can be calculated by kernel side during mount.
> 
> Signed-off-by: Li Dongyang <dongyangli@....com>

Applied, but I had to fix up a number of spelling errors in the commit
description, and fix up a large number of test failures caused by this
change.

	     	     	  	       	  - Ted

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