On 05/07/21 15:48, Martin Wilck wrote:
On Mo, 2021-07-05 at 15:11 +0200, Hannes Reinecke wrote:
On 7/5/21 3:02 PM, Paolo Bonzini wrote:
On 02/07/21 16:21, Martin Wilck wrote:
SG_IO gives you raw access to the SCSI logic unit, and you get
to
keep the pieces if anything goes wrong.
That's a very fragile user space API, on the fringe of being
useless
IMO.
Indeed. If SG_IO is for raw access to an ITL nexus, it shouldn't
be supported at all by mpath devices. If on the other hand SG_IO is
for raw access to a LUN, there is no reason for it to not support
failover.
Or we look at IO_URING_OP_URING_CMD, to send raw cdbs via io_uring.
And delegate SG_IO to raw access to an ITL nexus.
Doesn't help with existing issues, but should get a clean way
forward.
I still have to understand how this would help with the retrying
semantics. Wouldn't we get the exact same problem if a path error
occurs?
Also, how would the URING_CMD API differ from SG_IO modulo one being a
ioctl and one being io_uring-based? In the end what you have to do is
1) send a CDB and optionally some data 2) get back a status and
optionally some data and sense. Whether the intended use of the API is
for an ITL nexus or a LUN doesn't really matter. So, what is the
rationale for "SG_IO is for a nexus" in the first place, if you think
that "raw CDBs for a LUN" is a useful operation? You can still use
DM_TABLE_STATUS (iirc) to address a specific ITL nexus if desired.
Besides the virtualization case, think of udev rules that use SG_IO to
retrieve the device identification page for an mpath device and create
corresponding symlinks in /dev. They would fail if the first path is
not responding, which is not desirable.
Paolo
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