On 2018-01-09 11:45 AM, Christoph Hellwig wrote:
On Tue, Jan 09, 2018 at 10:07:28PM +0530, Kashyap Desai wrote:
Chris -
Overall NVME support behind MR controller is really a SCSI device. On top
of that, for MegaRaid, NVME device can be part of Virtual Disk and those
drive will not be exposed to the driver. User application may like to talk
to hidden NVME devices (part of VDs). This patch will extend the existing
interface for megaraid product in the same way it is currently supported
for other protocols like SMP, SATA pass-through.
Example - Current smartmon is using megaraid.h (MFI headers) to send SATA
pass-through.
https://github.com/mirror/smartmontools/blob/master/megaraid.h
And that is exactly the example of why we should have never allowed
megaraid any private passthrough ioctls to start with.
Christoph,
Have you tried to do any serious work with <linux/nvme_ioctl.h> and
say compared it with FreeBSD and Microsoft's approach? No prize for
guessing which one is worst (and least extensible). Looks like the
Linux pass-through was at the end of a ToDo list and was "designed"
at 5 a.m in the morning.
RAID cards need a pass-through that allows them to address one of
many physical disks behind the virtual disk presented to OS.
Pass-throughs need to have uncommited room for extra parameters that
will be passed through as-is to the RAID LLD.
So until Christoph gives an example of how that can be done with
<linux/nvme_ioctl.h> then I would like to see Christoph's objection
ignored.
And as a maintainer of smartmontools, I would like to point out that
pretty well all supported RAIDs, on all platforms need specialized
pass-through code. Start by looking at os_linux.cpp and then at the
other OSes. And now smartmontools supports NVMe on most platforms
and at the pass-through level, it is just another one, and not a
particularly clean one.
IMO Intel had their chance on the pass-through front, and blew it.
It is now too late to fix it and that job (impossible ?) should not
fall to MegaRaid maintainers.
Douglas Gilbert
