Re: [PATCH RFC 5/6] fs: xfs: iomap atomic write support

[John Garry <john.g.garry@oracle.com>]

>>>
>>> Not sure why we care about the file position, it's br_startblock that
>>> gets passed into the bio, not br_startoff.
>>
>> We just want to ensure that the length of the write is valid w.r.t. to the
>> offset within the extent, and br_startoff would be the offset within the
>> aligned extent.
> 
> Yes, I understand what br_startoff is, but this doesn't help me
> understand why this code is necessary.  Let's say you have a device that
> supports untorn writes of 16k in length provided the LBA of the write
> command is also aligned to 16k, and the fs has 4k blocks.
> 
> Userspace issues an 16k untorn write at offset 13k in the file, and gets
> this mapping:
> 
> [startoff: 13k, startblock: 16k, blockcount: 16k]
> 
> Why should this IO be rejected? 

It's rejected as it does not follow the rules.

> The physical space extent satisfies the
> alignment requirements of the underlying device, and the logical file
> space extent does not need aligning at all.

Sure. In this case, we can produce a single BIO and the underlying HW 
may be able to handle this atomically.

The point really is that we want a consistent userspace experience. We 
say that the write 'must' be naturally aligned, not 'should' be.

It's not really useful to the user if sometimes a write passes and 
sometimes it fails by chance of how the extents happen to be laid out.

Furthermore, in this case, what should the user do if this write at 13K 
offset fails as the 16K of data straddles 2x extents? They asked for 16K 
written at offset 13K and they want it done atomically - there is 
nothing which the FS can do to help. If they don't really need 16K 
written atomically, then better just do a regular write, or write 
individual chunks atomically.

Thanks,
John