linux-ext4 - Re: [PATCH v3 12/13] docs: Add fast commit documentation

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Message-ID: <20191018015655.GB21137@mit.edu>
Date:   Thu, 17 Oct 2019 21:56:55 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Harshad Shirwadkar <harshadshirwadkar@...il.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH v3 12/13] docs: Add fast commit documentation

On Tue, Oct 01, 2019 at 12:41:01AM -0700, Harshad Shirwadkar wrote:
> +
> +Multiple fast commit blocks are a part of one sub-transaction. To
> +indicate the last block in a fast commit transaction, fc_flags field
> +in the last block in every subtransaction is marked with "LAST" (0x1)
> +flag. A subtransaction is valid only if all the following conditions
> +are met:
> +
> +1) SUBTID of all blocks is either equal to or greater than SUBTID of
> +   the previous fast commit block.
> +2) For every sub-transaction, last block is marked with LAST flag.
> +3) There are no invalid blocks in between.

I'm wondering why we need to support multiple inodes being modified in
a single transaction.  As we currently have defined what can be done,
all updates to an inode should be free standing and not dependent on a
change to another inode, right?  And today, one block only modifies
one inode.

The only reason why we might want to define a sub-transaction as being
composed of multiple inodes, which must all be updated in an
all-or-nothing fashion, is the swap boot inode ioctl, and if that's
the only one, I wonder if it's worth the extra complexity.

Am I missing anything?

					- Ted