Re: [RFC 01/12] common/preamble: Fix fsx for ext4 with bigalloc

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

On Fri, Jun 13, 2025 at 11:01:25AM +0530, Ojaswin Mujoo wrote:
> On Thu, Jun 12, 2025 at 07:36:14AM -0700, Darrick J. Wong wrote:
> > On Thu, Jun 12, 2025 at 11:41:16AM +0530, Ojaswin Mujoo wrote:
> > > On Wed, Jun 11, 2025 at 07:30:05AM -0700, Darrick J. Wong wrote:
> > > > On Wed, Jun 11, 2025 at 03:04:44PM +0530, Ojaswin Mujoo wrote:
> > > > > From: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
> > > > > 
> > > > > Insert range and collapse range only works with bigalloc in case
> > > > > the range is cluster size aligned, which fsx doesnt take care. To
> > > > > work past this, disable insert range and collapse range on ext4, if
> > > > > bigalloc is enabled.
> > > > 
> > > > Hmmm, insert/collapse-range have the same behavior on xfs realtime,
> > > > maybe we should amend test() in fsx to round to the allocation unit
> > > > size?
> > > Hey Darrick,
> > > 
> > > Yes makes sense but as you mentioned, I'm not sure if there
> > > is a way to programatically detect the bigalloc cluster size (or
> > > allocation unit in general) like we do for xfs. 
> > 
> > I don't either, but maybe we should have a way reveal the allocation
> > unit size for a given file?  Yet another statx field? :P
> > 
> > (It /would/ be useful for programs that use collapse/insert range)
> 
> Yes it would, at the very least, help with defining clear semantics for
> collapse/insert range with bigalloc/rtvol because right now those
> operations just EINVAL if the range is not aligned correctly, which is
> confusing since it is not documented how to do it properly.
> 
> xfs does have an ioctl to get the geometry for rtvol. I think you are
> suggesting a more generic statx field which can be used by other FSes as
> well, right?

Right, since other filesystems (fat, ntfs, etc) also have allocation
units larger than the fsblock size.  Most of the time the allocunit
amplification simply doesn't matter to applications, but once in a while
it does (collapse/insert range, cow) affect performance.

--D

> 
> Regards,
> ojaswin
> > 
> > --D
> > 
>