linux-ext4 - Re: [PATCH] resize2fs: fix over-pessimistic heuristic.

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Date:	Thu, 10 Apr 2014 23:39:15 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] resize2fs: fix over-pessimistic heuristic.

On Sun, Apr 06, 2014 at 04:56:05PM +0400, Dmitry Monakhov wrote:
> In worst case we need one extent per moved block. Number of blocks to
> be moved is less or equals to blks_needed.

I can believe that the original safety margin of 0.2% of how much we
will be shrinking the file system might have been overly pessimistic.

However I'm concerned that your change might be over-optimistic.  For
example, if we are shrinking the file system down to under 500 blocks,
then the safety_margin will be 0 --- but it's could very easily be the
case that an extent tree will need to grow.

I'd accept this change, although I don't know it would make a
difference in the cases you are concerned about:

		blk64_t safe_margin = (ext2fs_blocks_count(fs->super) -
				       blks_needed)/500;
		if (safe_margin > blks_needed)
			safe_margin = blks_needed;

How and why are you using this?  I've never been all that exicited
about the -M option, since it's been abused in so many different ways,
and the result when you compress the file system that significantly is
often no good for performance.

					- Ted

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