Re: Block size and read-modify-write

[Carlos Maiolino <cmaiolino@redhat.com>]

On Fri, Dec 29, 2017 at 12:14:14AM +0100, Gionatan Danti wrote:
> Hi list,
> I would ask a question: how XFS block size affect read-modify-write in case
> of very small writes?
> 

Hi,

> For example, suppose an XFS filesystem with the default 4K data block size.
> Am I correct saying that:
> - a 512B normal, cached writes will cause a read-modify-write of the entire
> 4K sector?
> - a 512B O_DIRECT write will *not* cause a read-modify-write of the 4K
> sector, rather it will be flushed to disk as-is (512 bytes in length)?
> 

IIRC, although the DIO requirement is to have writes aligned to the logical sector
size, issuing such IOs not properly aligned with the filesystem block size, have
a few consequences.

- It will require exclusive inode io locks, so serializing IOs to the inode
- And yes, it will require a RMW to the block in question, all IO are always
  made in filesystem block size units.

I'm probably missing something else here though, but these two are the things I
had in my mind.

Cheers

> Thanks.
> 
> -- 
> Danti Gionatan
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-- 
Carlos