linux-ext4 - Re: discard and data=writeback

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Message-ID: <X+OIiNOGKmbwITC3@mit.edu>
Date:   Wed, 23 Dec 2020 13:12:24 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: discard and data=writeback

On Wed, Dec 23, 2020 at 01:47:33AM +0100, Matteo Croce wrote:
> As an extra test I extracted the archive with data=ordered, remounted
> with data=writeback and timed the rm -rf and viceversa.
> The mount option is the one that counts, the one using during
> extraction doesn't matter.

Hmm... that's really surprising.  At this point, the only thing I can
suggest is to try using blktrace to see what's going on at the block
layer when the I/O's and discard requests are being submitted.  If
there are no dirty blocks in the page cache, I don't see how
data=ordered vs data=writeback would make a difference to how mount -o
discard processing would take place.

Cheers,

					- Ted