Re: [PATCH v8 15/15] xfs: allow sysadmins to specify a maximum atomic write limit at mount time

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Apr 23, 2025 at 10:32:09AM +0200, Christoph Hellwig wrote:
> On Tue, Apr 22, 2025 at 12:27:39PM +0000, John Garry wrote:
> > From: "Darrick J. Wong" <djwong@kernel.org>
> > 
> > Introduce a mount option to allow sysadmins to specify the maximum size
> > of an atomic write.  If the filesystem can work with the supplied value,
> > that becomes the new guaranteed maximum.
> > 
> > The value mustn't be too big for the existing filesystem geometry (max
> > write size, max AG/rtgroup size).  We dynamically recompute the
> > tr_atomic_write transaction reservation based on the given block size,
> > check that the current log size isn't less than the new minimum log size
> > constraints, and set a new maximum.
> > 
> > The actual software atomic write max is still computed based off of
> > tr_atomic_ioend the same way it has for the past few commits.
> 
> The cap is a good idea, but a mount option for something that has
> strong effects for persistent application formats is a little suboptimal.
> But adding a sb field and an incompat bit wouldn't be great either.
> 
> Maybe this another use case for a trusted xattr on the root inode like
> the autofsck flag?

That would be even better, since you could set it at mkfs time and it
would persist until the next xfs_property set call.

--D