Re: [RFC PATCH 0/5] Enable use of Solid State Hybrid Drives

[Dave Chinner <david@fromorbit.com>]

On Wed, Oct 29, 2014 at 11:23:38AM -0700, Jason B. Akers wrote:
> The following series enables the use of Solid State hybrid drives
> ATA standard 3.2 defines the hybrid information feature, which provides a means for the host driver to provide hints to the SSHDs to guide what to place on the SSD/NAND portion and what to place on the magnetic media.
> 
> This implementation allows user space applications to provide the cache hints to the kernel using the existing ionice syscall. 
> 
> An application can pass a priority number coding up bits 11, 12, and 15 of the ionice command to form a 3 bit field that encodes the following priorities:
> 	OPRIO_ADV_NONE,
> 	IOPRIO_ADV_EVICT, /* actively discard cached data */
> 	IOPRIO_ADV_DONTNEED, /* caching this data has little value */
> 	IOPRIO_ADV_NORMAL, /* best-effort cache priority (default) */
> 	IOPRIO_ADV_RESERVED1, /* reserved for future use */
> 	IOPRIO_ADV_RESERVED2,
> 	IOPRIO_ADV_RESERVED3,
> 	IOPRIO_ADV_WILLNEED, /* high temporal locality */
> 
> For example the following commands from the user space will make dd IOs to be generated with a hint of IOPRIO_ADV_DONTNEED assuming the SSHD is /dev/sdc.
> 
> ionice -c2 -n4096 dd if=/dev/zero of=/dev/sdc bs=1M count=1024
> ionice -c2 -n4096 dd if=/dev/sdc of=/dev/null bs=1M count=1024

This looks to be the wrong way to implement per-IO priority
information.

How does a filesystem make use of this to make sure it's
metadata ends up with IOPRIO_ADV_WILLNEED to store frequently
accessed metadata in flash. Conversely, journal writes need to
be issued with IOPRIO_ADV_DONTNEED so they don't unneceessarily
consume flash space as they are never-read IOs...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com