On Wed, Oct 29, 2014 at 1:14 PM, Dave Chinner <david@xxxxxxxxxxxxx> 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... Internally this is still using the prio bits in bio->rw. So a filesystem should be able to use bio_set_prio() if it wants to override the default priority from userspace. -- 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
