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I am >> concerned of prospect of heavily fragmented files, though. > Yes that's true, if the FS is up long enough there is bound to be > fragmentation eventually which might make it harder for extsize to > get the blocks. > > With software fallback, there's again the point that many FSes will need > some sort of COW/exchange_range support before they can support anything > like that. > > Although I;ve not looked at what it will take to add that to > ext4 but I'm assuming it will not be trivial at all. Sure, but then again you may not have issues with getting forcealign support accepted for ext4. However, I would have thought that bigalloc was good enough to use initially. > >>> I agree that forcealign is not the only way we can have atomic writes >>> work but I do feel there is value in having forcealign for FSes and >>> hence we should have a discussion around it so we can get the interface >>> right. >>> >> I thought that the interface for forcealign according to the candidate xfs >> implementation was quite straightforward. no? > As mentioned in the original proposal, there are still a open problems > around extsize and forcealign. > > - The allocation and deallocation semantics are not completely clear to > me for example we allow operations like unaligned punch_hole but not > unaligned insert and collapse range, and I couldn't see that > documented anywhere. For xfs, we were imposing the same restrictions as which we have for rtextsize > 1. If you check the following: https://lore.kernel.org/linux-xfs/20240813163638.3751939-9-john.g.garry@oracle.com/ You can see how the large allocunit value is affected by forcealign, and then check callers of xfs_is_falloc_aligned() -> xfs_inode_alloc_unitsize() to see how this affects some fallocate modes. > > - There are challenges in extsize with delayed allocation as well as how > the tooling should handle forcealigned inodes. Yeah, maybe. I was only testing my xfs forcealign solution for dio (and no delayed alloc). > > - How are FSes supposed to behave when forcealign/extsize is used with > other FS features that change the allocation granularity like bigalloc > or rtvol. As you would expect, they need to be aligned with one another. For example, in the case of xfs rtvol, rextsize needs to be a multiple of extsize when forcealign is enabled. Or the other way around, I forget now.. > > I agree that XFS's implementation is a good reference but I'm > sure as I continue working on the same from ext4 perspective we will have > more points of discussion. So I definitely feel that its worth > discussing this at LSFMM. Understood, but I wait to see what happens to my CoW-based method for XFS to see where that goes before commenting on what needs to be discussed for xfs > >> What was not clear was the age-old issue of how to issue an atomic write of >> mixed extents, which is really an atomic write issue. > Right, btw are you planning any talk for atomic writes at LSFMM? I hadn't planned on it, but I guess that Martin will add something to the agenda. Thanks, John