Re: Block size and read-modify-write

[Dave Chinner <david@fromorbit.com>]

On Tue, Jan 02, 2018 at 11:25:39AM +0100, Carlos Maiolino wrote:
> 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?

Cached writes smaller than a *page* will cause RMW cycles in the
page cache, regardless of the block size of the filesystem.

> > - 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)?

Ok, there is a difference between *sector size* and *filesystem
block size*. You seem to be using them interchangably in your
question, and that's not correct.

> 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

That is correct, but....

> - And yes, it will require a RMW to the block in question, all IO are always
>   made in filesystem block size units.

.... this is not correct for direct IO. The direct IO path does not
do RMW cycles at all.

Put simply: a 512B DIO write on a (real or emulated) 512B sector
device with a 4k FSB will be serialised by the filesystem and do a
single 512B sector write to the device.  However, if the device
reports as a 4k sector device then a 512B DIO write will be rejected
by the filesystem because sub-sector IO is not possible.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com