On 10/29/2014 02:14 PM, Dave Chinner wrote: > 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... Not disagreeing that loading more into the io priority fields is a bit... icky. I see why it's done, though, it requires the least amount of plumbing. As for the fs accessing this, the io nice fields are readily exposed through the ->bi_rw setting. So while the above example uses ionice to set a task io priority (that a bio will then inherit), nothing prevents you from passing it in directly from the kernel. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
