Messages in this thread | | | Date | Thu, 30 Oct 2014 07:14:17 +1100 | From | Dave Chinner <> | Subject | Re: [RFC PATCH 0/5] Enable use of Solid State Hybrid Drives |
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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
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