Re: [PATCH v2 0/7] btrfs-progs: use direct-IO for zoned device

From: David Sterba <dsterba@suse.cz>
To: Naohiro Aota <naohiro.aota@wdc.com>
Cc: linux-btrfs@vger.kernel.org, David Sterba <dsterba@suse.com>
Subject: Re: [PATCH v2 0/7] btrfs-progs: use direct-IO for zoned device
Date: Wed, 6 Oct 2021 23:02:47 +0200	[thread overview]
Message-ID: <20211006210247.GY9286@twin.jikos.cz> (raw)
In-Reply-To: <20211005062305.549871-1-naohiro.aota@wdc.com>

On Tue, Oct 05, 2021 at 03:22:58PM +0900, Naohiro Aota wrote:
> As discussed in the Zoned Storage page [1],  the kernel page cache does not
> guarantee that cached dirty pages will be flushed to a block device in
> sequential sector order. Thus, we must use O_DIRECT for writing to a zoned
> device to ensure the write ordering.
> 
> [1] https://zonedstorage.io/linux/overview/#zbd-support-restrictions
> 
> As a writng buffer is embedded in some other struct (e.g., "char data[]" in
> struct extent_buffer), it is difficult to allocate the struct so that the
> writng buffer is aligned.
> 
> This series introduces btrfs_{pread,pwrite} to wrap around pread/pwrite,
> which allocates an aligned bounce buffer, copy the buffer contents, and
> proceeds the IO. And, it now opens a zoned device with O_DIRECT.
> 
> Since the allocation and copying are costly, it is better to do them only
> when necessary. But, it is cumbersome to call fcntl(F_GETFL) to determine
> the file is opened with O_DIRECT or not every time doing an IO.
> 
> As zoned device forces to use zoned btrfs, I decided to use the zoned flag
> to determine if it is direct-IO or not. This can cause a false-positive (to
> use the bounce buffer when a file is *not* opened with O_DIRECT) in case of
> emulated zoned mode on a non-zoned device or a regular file. Considering
> the emulated zoned mode is mostly for debugging or testing, I believe this
> is acceptable.
> 
> * Changes
> v2
>   - Rebased on the latest "devel" branch
>   - Add patch to fix segfault in several cases
>   - drop ZONED flag from BTRFS_CONVERT_ALLOWED_FEATURES
> 
> Patches 1 to 3 are preparation to fix some issues in the current code.
> 
> Patches 4 and 5 wraps pread/pwrite with newly introduced function
> btrfs_pread/btrfs_pwrite.
> 
> Patch 6 deals with the zoned flag while reading the initial trees.
> 
> Patch 7 finally opens a zoned device with O_DIRECT.
> 
> Naohiro Aota (7):
>   btrfs-progs: mkfs: do not set zone size on non-zoned mode
>   btrfs-progs: set eb->fs_info properly
>   btrfs-progs: drop ZONED flag from BTRFS_CONVERT_ALLOWED_FEATURES
>   btrfs-progs: introduce btrfs_pwrite wrapper for pwrite
>   btrfs-progs: introduce btrfs_pread wrapper for pread
>   btrfs-progs: temporally set zoned flag for initial tree reading
>   btrfs-progs: use direct-io for zoned device

Is this still supposed to work?

  $ ./mkfs.btrfs -f -O zoned -d single -m single img
  ...
  ERROR: 16384 is not aligned to 1048576
  ERROR: error during mkfs: Input/output error

On commit below this patchset it works and creates a filesystem with
zoned mode and zone size 256M.