linux-ext4 - Re: [PATCH] ext4: attempt to shrink directory on dentry removal

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Date:   Mon, 26 Aug 2019 22:38:48 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     harshad shirwadkar <harshadshirwadkar@...il.com>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: attempt to shrink directory on dentry removal

On Mon, Aug 26, 2019 at 02:46:01PM -0700, harshad shirwadkar wrote:
> By this method we end up reading up to 2 extra blocks (one previous
> and one next) that are not going to be merged. That's the trade-off we
> have to make in order to avoid any changes to on-disk structure (If we
> modify the on-disk structure and store the fullness in the dx block,
> we would read only the blocks that need to be merged).

We can also adjust the merging strategy depending on whether the
previous and/or next blocks are in memory.  If they are in memory,
that we might try merging if the block is < 50% full.  If they are not
in memory, it might not be worth doing the read until the block is
empty, or maybe, say, 10% full.

> Since merging approach also requires a way to free up directory
> blocks, I think we could first get a patch in that can free up
> directory blocks by swapping with the last block. Once we have that
> then we could implement merging.

I agree; I'd wait to implementing merging until we get directory block
removal working.  Simply trying to shrink the directory when a leaf
block which is *not* the last block in an indexed directory is going
to be substantially better compared to waiting until the last block in
the directory is empty.

						- Ted