On 4/27/21 12:52 PM, Ulrich Windl wrote:
Hannes Reinecke <hare@xxxxxxx> schrieb am 27.04.2021 um 10:21 in Nachricht
<2a6903e4-ff2b-67d5-e772-6971db8448fb@xxxxxxx>:
On 4/27/21 10:10 AM, Martin Wilck wrote:
On Tue, 2021‑04‑27 at 13:48 +1000, Erwin van Londen wrote:
Wrt 1), we can only hope that it's the case. But 2) and 3) need work,
afaics.
In my view the WWID should never change.
In an ideal world, perhaps not. But in the dm‑multipath realm, we know
that WWID changes can happen with certain storage arrays. See
https://listman.redhat.com/archives/dm‑devel/2021‑February/msg00116.html
and follow‑ups, for example.
And it's actually something which might happen quite easily.
The storage array can unmap a LUN, delete it, create a new one, and map
that one into the same LUN number than the old one.
If we didn't do I/O during that interval upon the next I/O we will be
getting the dreaded 'Power‑On/Reset' sense code.
_And nothing else_, due to the arcane rules for sense code generation in
SAM.
But we end up with a completely different device.
The only way out of it is to do a rescan for every POR sense code, and
disable the device eg via DID_NO_CONNECT whenever we find that the
identification has changed. We already have a copy of the original VPD
page 0x83 at hand, so that should be reasonably easy.
I don't know the depth of the SCSI or FC protocol, but storage systems
typically signal such events, maybe either via some unit attention or some FC
event. Older kernels logged that there was a change, but a manual SCSI bus scan
is needed, while newer kernels find new devices "automagically" for some
products. The HP EVA 6000 series wored that way, a 3PAR SotorServ 8000 series
also seems to work that way, but not Pure Storage X70 R3. FOr the latter you
need something like a FC LIP to make the kernel detect the new devices (LUNs).
I'm unsure where the problem is, but in principle the kernel can be
notified...
My point was that while there _is_ a unit attention with the sense code
'INQUIRY DATA CHANGED' (and that indeed will generate a kernel message),
it might be obscured by a subsequent unit attention with the sense code
'Power-On/Reset', as per SCSI spec the latter might cause the previous
ones to _not_ being sent.
So from that reasoning we will need to rescan the device upon
'Power-on/Reset'.
But 'Power-On/Reset' is a sense code which we also get during initial
device scan, so the problem is that we will be triggering a rescan while
_doing_ a rescan, and as such it would need some really careful testing.
As for the PureStorage behaviour: The problem with changing the LUN
mapping on the array is that it we might not _have_ a device to send
unit attentions to.
If the array already exports LUNs to some other hosts, it doesn't need
to re-initialize the FC port when starting to export LUNs to _this_
host. And as _this_ host doesn't have a LUN on which unit attentions can
be sent, _and_ the FC port is already registered, there are no events
whatsoever which would cause the host to initiate a rescan.
To resolve that the array would need to induce eg an RSCN, but that will
only be triggered if a FC port is (re-)registered.
Which is what HPe arrays do; initiate a link-bounce on the attached
ports, which will cause the attached hosts to initiate a rescan.
Of course, _all_ hosts will need to rescan (and thereby causing an
interruption even on unrelated hosts), which is why this is not done by
all vendors.
I had a rather lengthy discussion with Fred Knight @ NetApp about
Power‑On/Reset handling, what with him complaining that we don't handle
is correctly. So this really is something we should be looking into,
even independently of multipathing.
But actually I like the idea from Martin Petersen to expose the parsed
VPD identifiers to sysfs; that would allow us to drop sg_inq completely
from the udev rules.
Talking of VPDs: Somewhere in the last 12 years (within SLES 11)there was a
kernel change regarding trailing blanks in VPD data. That change blew up
several configurations being unable to re-recognize the devices. In one case
the software even had bound a license to a specific device with serial number,
and that software found "new" devices while missing the "old" ones...
That's probably just for VPD page 0x80. Page 0x83 has pretty strict
rules on how the entries are formatted, so chopping off trailing blanks
is not easily done there.
Cheers,
Hannes
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Dr. Hannes Reinecke Kernel Storage Architect
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