Re: [PATCH v4 00/14] forcealign for xfs

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

On Mon, Sep 16, 2024 at 11:24:56AM +0100, John Garry wrote:
> On 16/09/2024 08:03, Dave Chinner wrote:
> > OTOH, we can't do this with atomic writes. Atomic writes require
> > some mkfs help because they require explicit physical alignment of
> > the filesystem to the underlying storage.

Forcealign requires agsize%extsize==0, it's atomicwrites that adds the
requirement that extsize be a power of 2...

> If we are enabling atomic writes at mkfs time, then we can ensure agsize %
> extsize == 0. That provides the physical alignment guarantee. It also makes
> sense to ensure extsize is a power-of-2.
> 
> However, extsize is re-configurble per inode. So, for an inode enabled for
> atomic writes, we must still ensure agsize % new extsize == 0 (and also new
> extsize is a power-of-2)

...so yes.

> > Hence we'll eventually end
> > up with atomic writes needing to be enabled at mkfs time, but force
> > align will be an upgradeable feature flag.
> 
> Could atomic writes also be an upgradeable feature? We just need to ensure
> that agsize % extsize == 0 for an inode enabled for atomic writes. Valid
> extsize values may be quite limited, though, depending on the value of
> agsize.

I don't see why forcealign and atomicwrites can't be added to the sb
featureset after the fact, though you're right that callers of xfs_io
chattr might be hard pressed to find an fsx_extsize value that fits.

--D

> Thanks,
> John
> 
>