Re: [RFC PATCH 1/7] statx: add I/O alignment information

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

On 2/10/22 10:11 PM, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Traditionally, the conditions for when DIO (direct I/O) is supported
> were fairly simple: filesystems either supported DIO aligned to the
> block device's logical block size, or didn't support DIO at all.
> 
> However, due to filesystem features that have been added over time (e.g,
> data journalling, inline data, encryption, verity, compression,
> checkpoint disabling, log-structured mode), the conditions for when DIO
> is allowed on a file have gotten increasingly complex.  Whether a
> particular file supports DIO, and with what alignment, can depend on
> various file attributes and filesystem mount options, as well as which
> block device(s) the file's data is located on.
> 
> XFS has an ioctl XFS_IOC_DIOINFO which exposes this information to
> applications.  However, as discussed
> (https://lore.kernel.org/linux-fsdevel/20220120071215.123274-1-ebiggers@kernel.org/T/#u),
> this ioctl is rarely used and not known to be used outside of
> XFS-specific code.  It also was never intended to indicate when a file
> doesn't support DIO at all, and it only exposes the minimum I/O
> alignment, not the optimal I/O alignment which has been requested too.
> 
> Therefore, let's expose this information via statx().  Add the
> STATX_IOALIGN flag and three fields associated with it:
> 
> * stx_mem_align_dio: the alignment (in bytes) required for user memory
>    buffers for DIO, or 0 if DIO is not supported on the file.
> 
> * stx_offset_align_dio: the alignment (in bytes) required for file
>    offsets and I/O segment lengths for DIO, or 0 if DIO is not supported
>    on the file.  This will only be nonzero if stx_mem_align_dio is
>    nonzero, and vice versa.
> 
> * stx_offset_align_optimal: the alignment (in bytes) suggested for file
>    offsets and I/O segment lengths to get optimal performance.  This
>    applies to both DIO and buffered I/O.  It differs from stx_blocksize
>    in that stx_offset_align_optimal will contain the real optimum I/O
>    size, which may be a large value.  In contrast, for compatibility
>    reasons stx_blocksize is the minimum size needed to avoid page cache
>    read/write/modify cycles, which may be much smaller than the optimum
>    I/O size.  For more details about the motivation for this field, see
>    https://lore.kernel.org/r/20220210040304.GM59729@dread.disaster.area
> 
> Note that as with other statx() extensions, if STATX_IOALIGN isn't set
> in the returned statx struct, then these new fields won't be filled in.
> This will happen if the filesystem doesn't support STATX_IOALIGN, or if
> the file isn't a regular file.  (It might be supported on block device
> files in the future.)  It might also happen if the caller didn't include
> STATX_IOALIGN in the request mask, since statx() isn't required to
> return information that wasn't requested.
> 
> This commit adds the VFS-level plumbing for STATX_IOALIGN.  Individual
> filesystems will still need to add code to support it.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---


I've actually worked on similar series to export alignment and 
granularity for non-trivial operations, this implementation
only exporting I/O alignments (mostly REQ_OP_WRITE/REQ_OP_READ) via
stax.

Since it is coming from :-
bdev_logical_block_size()->q->limits.logical_block_size that is set when
low level driver like nvme calls blk_queue_logical_block_size().

 From my experience especially with SSDs, applications want to
know similar information about different non-trivial requests such as
REQ_OP_DISCARD/REQ_OP_WRITE_ZEROES/REQ_OP_VERIFY (work in progress see
[1]) etc.

It will be great to make this generic userspace interface where user can
ask for specific REQ_OP_XXX such as generic I/O REQ_OP_READ/REQ_OP_WRITE
and non generic REQ_OP_XX such as REQ_OP_DISCARD/REQ_OP_VERIFY etc ....

Since I've worked on implementing REQ_OP_VERIFY support I don't want to
implement separate interface for querying the REQ_OP_VERIFY or any other
non-trivial REQ_OP_XXX granularity or alignment.

-ck

[1] https://www.spinics.net/lists/linux-xfs/msg56826.html