Re: [PATCH 0/5] btrfs-progs: use direct-IO for zoned device

[Naohiro Aota <Naohiro.Aota@wdc.com>]

On Mon, Sep 27, 2021 at 09:26:18PM +0200, David Sterba wrote:
> On Mon, Sep 27, 2021 at 01:15:49PM +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.
> 
> This should be in the changelog somewhere too, the last patch looks like
> a good place so I'll copy it there.
> 
> > 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.
> 
> Agreed.
> 
> All patches added to devel. Would be good to add some tests for the
> emulated mode, ie. that we can test at least something regularly without
> special devices.

Will do. We may also add some tests for zoned device by setting up
null_blk (provided the machine has enough memory).