[dm-devel] [LSF/MM TOPIC][ATTEND] Plumbing I/O Cache / Tiered Storage Hints

[Dan Williams]

On Tue, Jan 28, 2014 at 2:57 PM, Dan Williams <dan.j.williams at intel.com> wrote:
> In addition to disk drives with internally tiered-storage (solid-state
> + magnetic media) the kernel has also grown native I/O-caching /
> tiered-storage implementations in bcache and dm-cache.
>
> Currently, all these solutions depend on heuristics to determine what
> tier the data referenced in an I/O belongs.  However, the presence of
> hinting proposals from SCSI [1], ATA [2], and bcache [3] indicate that
> these devices (hardware or virtual) want to consume explicit hints
> indicating the value of caching data in a higher performing tier.
>
> At LSF I want to discuss options and opportunities for plumbing cache
> hints from userspace, through the I/O stack to devices.  My colleague,
> Jason Akers, is also interested in this discussion as he is
> investigating how to exploit these hints from userspace.  I
> participated in the LSF 2012 discussion of this topic, see that it was
> raised again at LSF 2013, and note that we have not settled on an
> enabling path.  What's new for this year is an effort to set aside,
> for now, the deeper complexities of the device specification proposals
> and focus on a minimal set of hints that can be specified per-process
> (ionice) and maybe per-file (fadvise).
>
> My suspicion is that AIO attributes is useful for applications that
> want access to the full range of access hints exposed by a device.
> However, for the general buffered-I/O / tiered-storage case, a small
> set of hints achieves the bulk of the value.
>
> I am also interested in:
> Volatile ranges
> SMR Enabling
> Integrity passthrough
>
> --
> Dan
>
> [1] T10 LBA Access Hints
> [2] T13 Hybrid Information Feature

Correction, the hinting scheme is defined by SATA-IO in SATA 3.2

> [3] AIO Attributes: http://marc.info/?l=linux-aio&m=136580574523674&w=2