9 a.m.
On Wed, Apr 26, 2017 at 1:38 PM, Jeff Moyer <jmoyer(a)redhat.com> wrote:
Dan Williams <dan.j.williams(a)intel.com> writes:
> The nvdimm_flush() mechanism helps to reduce the impact of an ADR
> (asynchronous-dimm-refresh) failure. The ADR mechanism handles flushing
> platform WPQ (write-pending-queue) buffers when power is removed. The
> nvdimm_flush() mechanism performs that same function on-demand.
>
> When a pmem namespace is associated with a block device, an
> nvdimm_flush() is triggered with every block-layer REQ_FUA, or REQ_FLUSH
> request. These requests are typically associated with filesystem
> metadata updates. However, when a namespace is in device-dax mode,
> userspace (think database metadata) needs another path to perform the
> same flushing. In other words this is not required to make data
> persistent, but in the case of metadata it allows for a smaller failure
> domain in the unlikely event of an ADR failure.
>
> The new 'flush' attribute is visible when the individual DIMMs backing a
> given interleave-set are described by platform firmware. In ACPI terms
> this is "NVDIMM Region Mapping Structures" and associated "Flush Hint
> Address Structures". Reads return "1" if the region supports
triggering
> WPQ flushes on all DIMMs. Reads return "0" the flush operation is a
> platform nop, and in that case the attribute is read-only.
I can make peace with exposing this to userspace, though I am mostly
against its use. However, sysfs feels like the wrong interface.
Believe it or not, I'd rather see this implemented as an ioctl.
This isn't a NACK, it's me giving my opinion. Do with it what you will.
I hate ioctls with a burning passion so I can't get on board with that
change, but perhaps the sentiment behind it is that this is too
visible and too attractive being called "flush" in sysfs? Would a name
more specific to the mechanism make it more palatable? Like
"flush_hint_trigger" or "wpq_drain"?