Date:   Mon, 18 Sep 2023 08:05:20 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Pankaj Raghav <kernel@...kajraghav.com>
Cc:     linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        p.raghav@...sung.com, da.gomez@...sung.com,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        willy@...radead.org, djwong@...nel.org, linux-mm@...ck.org,
        chandan.babu@...cle.com, mcgrof@...nel.org, gost.dev@...sung.com
Subject: Re: [RFC 00/23] Enable block size > page size in XFS

On Fri, Sep 15, 2023 at 08:38:25PM +0200, Pankaj Raghav wrote:
> From: Pankaj Raghav <p.raghav@...sung.com>
> 
> There has been efforts over the last 16 years to enable enable Large
> Block Sizes (LBS), that is block sizes in filesystems where bs > page
> size [1] [2]. Through these efforts we have learned that one of the
> main blockers to supporting bs > ps in fiesystems has been a way to
> allocate pages that are at least the filesystem block size on the page
> cache where bs > ps [3]. Another blocker was changed in filesystems due to
> buffer-heads. Thanks to these previous efforts, the surgery by Matthew
> Willcox in the page cache for adopting xarray's multi-index support, and
> iomap support, it makes supporting bs > ps in XFS possible with only a few
> line change to XFS. Most of changes are to the page cache to support minimum
> order folio support for the target block size on the filesystem.
> 
> A new motivation for LBS today is to support high-capacity (large amount
> of Terabytes) QLC SSDs where the internal Indirection Unit (IU) are
> typically greater than 4k [4] to help reduce DRAM and so in turn cost
> and space. In practice this then allows different architectures to use a
> base page size of 4k while still enabling support for block sizes
> aligned to the larger IUs by relying on high order folios on the page
> cache when needed. It also enables to take advantage of these same
> drive's support for larger atomics than 4k with buffered IO support in
> Linux. As described this year at LSFMM, supporting large atomics greater
> than 4k enables databases to remove the need to rely on their own
> journaling, so they can disable double buffered writes [5], which is a
> feature different cloud providers are already innovating and enabling
> customers for through custom storage solutions.
> 
> This series still needs some polishing and fixing some crashes, but it is
> mainly targeted to get initial feedback from the community, enable initial
> experimentation, hence the RFC. It's being posted now given the results from
> our testing are proving much better results than expected and we hope to
> polish this up together with the community. After all, this has been a 16
> year old effort and none of this could have been possible without that effort.
> 
> Implementation:
> 
> This series only adds the notion of a minimum order of a folio in the
> page cache that was initially proposed by Willy. The minimum folio order
> requirement is set during inode creation. The minimum order will
> typically correspond to the filesystem block size. The page cache will
> in turn respect the minimum folio order requirement while allocating a
> folio. This series mainly changes the page cache's filemap, readahead, and
> truncation code to allocate and align the folios to the minimum order set for the
> filesystem's inode's respective address space mapping.
> 
> Only XFS was enabled and tested as a part of this series as it has
> supported block sizes up to 64k and sector sizes up to 32k for years.
> The only thing missing was the page cache magic to enable bs > ps. However any filesystem
> that doesn't depend on buffer-heads and support larger block sizes
> already should be able to leverage this effort to also support LBS,
> bs > ps.
> 
> This also paves the way for supporting block devices where their logical
> block size > page size in the future by leveraging iomap's address space
> operation added to the block device cache by Christoph Hellwig [6]. We
> have work to enable support for this, enabling LBAs > 4k on NVME,  and
> at the same time allow coexistence with buffer-heads on the same block
> device so to enable support allow for a drive to use filesystem's to
> switch between filesystem's which may depend on buffer-heads or need the
> iomap address space operations for the block device cache. Patches for
> this will be posted shortly after this patch series.

Do you have a git tree branch that I can pull this from
somewhere?

As it is, I'd really prefer stuff that adds significant XFS
functionality that we need to test to be based on a current Linus
TOT kernel so that we can test it without being impacted by all
the random unrelated breakages that regularly happen in linux-next
kernels....

-Dave.
-- 
Dave Chinner
david@...morbit.com

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