Re: [PATCH v4 00/14] forcealign for xfs

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

> 
>> 1. As I understand XFS untorn atomic write support is built on top
>> of FORCEALIGN feature (which this series is adding) which in turn
>> uses extsize hint feature underneath.
> 
> Yes.
> 
>>     Now extsize hint mainly controls the alignment of both
>>     "physical start" & "logical start" offset and extent length,
>>     correct?
> 
> Yes.

To be clear, only for atomic writes do we require physical block alignment.

> 
>>     This is done using args->alignment for start aand
>>     args->prod/mode variables for extent length. Correct?
> 
> Yes.
> 
>>     - If say we are not able to allocate an aligned physical start?
>>     Then since extsize is just a hint we go ahead with whatever
>>     best available extent is right?

---

> 
>>
>>     - Does the unmapping of extents also only happens in extsize
>>     chunks (with forcealign)?
> 
> Yes, via use of xfs_inode_alloc_unitsize() in the high level code
> aligning the fsbno ranges to be unmapped.
> 
> Remember, force align requires both logical file offset and
> physical block number to be correctly aligned, so unmap alignment
> has to be set up correctly at file offset level before we even know
> what extents underly the file range we need to unmap....
> 
>>       If the start or end of the extent which needs unmapping is
>>       unaligned then we convert that extent to unwritten and skip,
>>       is it? (__xfs_bunmapi())
> 
> The high level code should be aligning the start and end of the
> file range to be removed via xfs_inode_alloc_unitsize(). 

Is that the case for something like truncate? There we just say what is 
the end block which we want to truncate to in 
xfs_itruncate_extents_flags(new_size)  -> 
xfs_bunmapi_range(XFS_B_TO_FSB(new_size)), and that may not be alloc 
unit aligned.

> Hence
> the low level __xfs_bunmapi() code shouldn't ever be encountering
> unaligned unmaps on force-aligned inodes.