On 2018-03-30 04:01 PM, Bart Van Assche wrote:
On Fri, 2018-03-30 at 12:36 -0600, Tim Walker wrote:
Yes I will be there to discuss the multi-LUN approach. I wanted to get
these interface details out so we could have some background and
perhaps folks would come with ideas. I don't have much more to put
out, but I will certainly answer questions - either on this list or
directly.
Hello Tim,
As far as I know the Linux SCSI stack does not yet support any SCSI devices
for which a single SCSI command affects multiple (two in this case) LUNs.
Adding such support may take significant work. There will also be the
disadvantage that most SCSI core contributors do not have access to a multi-
actuator device and hence that their changes may break support for multi-
actuator devices.
Hmmm, INQUIRY (3PC bit) and REPORT LUNS seem to be counter examples to
Bart's assertion. Plus there are a more that tell you about things outside
the addressed LU, for example the SCSI Ports VPD page tells you about other
SCSI ports hence other LUs) on the current device.
From Tim's command list:
Device level
------------
0x0, 0x1: okay
0x4 (Format unit): yikes, that could be a nasty surprise, accessing a file
system on the other LU and getting an error "Not ready, format in progress"!!
0x12: standard INQUIRY okay, VPD pages not so much LU id different; relative
port id, different; target port id different (at the least)
0x1b (SSU): storage LUs need to know this model, otherwise the logic on
each LU could get into a duel: "I want it spun up; no, I want it spun
down ..."
0x35, 0x37, 0x3b, 0x3c: okay
0x48 (sanitize): similar to Format unit above
0x91,0x4c,0x4d: okay
MODE SENSE/SELECT(6,10): depends on page, block descriptor needs to be
partially device level (since LB size may be changed by FU which is
device level)
rest of device level: okay or (I) don't know
0xf7: READ UDS DATA, that's interesting, but proprietary I guess
Perhaps you could add a rider on FU and SAN: they get rejected unless the
other storage LU is in (logical) spun down state.
LU specific
-----------
all okay, I hoping READ(6,10,12,16,32) and their WRITE cousins will be
there also :-) Plus the TMF: LU reset
Device or LU
------------
all okay
I'm intrigued by your 3 LU model. My wish list for that:
LUN 0 would be processor device type (0x3) so it wouldn't confuse the
OS (Linux) that it held storage (READ CAPACITY is mandatory for PDT 0x0
and cannot represent a 0 block LU) and you could pick and choose which
SCSI commands to implement on it. LUN 0 TUR could report what the spindle
was really doing, SSU could do precisely what it is told (and SSU on LUNs
1 and 2 would be an "and" for spin down and an "or" for spin up). I've
got several boxes full of SAS cables and only one cable that I can think
of that lets me get to the secondary SAS port. So on LUN 0 you could have
a proprietary (unless T10 takes it on board) mode page to enable the user
to say which ports that LUNs 1 and 2 where accessible on. Obviously LUN 0
would need to be accessible on both ports. [A non-accessible LUN would
still respond to INQUIRY but with a first byte of 07f: PDT=0x1f (unknown)
and PQ=3 which implies it is there but inaccessible via the current port.]
A processor PDT opens up the possibility of putting a copy manager on
LUN 0. Think offloaded (from main machine's perspective) backups and
restores where LUN 1 or 2 is the source or destination.
Enough dreaming.
Doug Gilbert
