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From: Phillip Susi <phill@thesusis.net>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-fsdevel@vger.kernel.org, Theodore Ts'o <tytso@mit.edu>
Subject: Re: Uneccesary flushes waking up suspended disks
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Dave Chinner <david@fromorbit.com> writes:

> Now the filesystem is idle, with no dirty data or metadata.
>
> In the case of XFS, this will begin the process of "covering the
> log". This takes 60-90s (3 consecutive log sync worker executions),
> and it involves the journal updating and logging the superblock and
> writing it back to mark the journal as empty.

Apparently ext4 only bothers journaling metadata updates.  If part of a
regular file data is overwritten, it does not trigger a transaction.
Are you saying that XFS does commit a transaction and therefore flush
after just overwriting an existing block in a file, with no metadata
being changed?