On 01/13/2016 11:50 AM, Sagi Grimberg wrote:
Hi all,
I'd like to attend LSF/MM and would like to present my ideas for a
multipath redesign.
The overall idea is to break up the centralized multipath handling in
device-mapper (and multipath-tools) and delegate to the appropriate
sub-systems.
I agree that would be very useful. Great topic. I'd like to attend
this talk as well.
Individually the plan is:
a) use the 'wwid' sysfs attribute to detect multipath devices;
this removes the need of the current 'path_id' functionality
in multipath-tools
CC'ing Linux-nvme,
I've recently looked at multipathing support for nvme (and nvme over
fabrics) as well. For nvme the wwid equivalent is the nsid (namespace
identifier). I'm wandering if we can have better abstraction for
user-space so it won't need to change its behavior for scsi/nvme.
The same applies for the the timeout attribute for example which
assumes scsi device sysfs structure.
My idea for this is to lookup the sysfs attribute directly from
multipath-tools. As such we would need to have some transport
information in multipath so that we know where to find it.
And with that we should easily able to accomodate NVMe, provided the
nsid is displayed somewhere in sysfs.
b) leverage topology information from scsi_dh_alua (which we will
have once my ALUA handler update is in) to detect the multipath
topology. This removes the need of a 'prio' infrastructure
in multipath-tools
This would require further attention for nvme.
Indeed. But then I'm not sure how multipath topology would be
represented in NVMe; we would need some way of transmitting the
topology information.
Easiest would be to leverage VPD device information; so we only need
the equivalent of REPORT TARGET PORT GROUPS to implement an
ALUA-like scenario.
c) implement block or scsi events whenever a remote port becomes
unavailable. This removes the need of the 'path_checker'
functionality in multipath-tools.
I'd prefer if we'd have it in the block layer so we can have it for all
block drivers. Also, this assumes that port events are independent of
I/O. This assumption is incorrect in SRP for example which detects port
failures only by I/O errors (which makes path sensing a must).
That's what I though initially, too.
But then we're facing a layering issue:
The path events are generated at the _transport_ level.
So for SCSI we have to do a redirection
transport layer->scsi layer->scsi ULD->block device
requiring us to implement for sets of callback functions.
Which I found rather pointless (and time consuming), so I opted for
scsi events (like we have for UNIT ATTENTION) instead.
However, even now we're having two sets of events (block events and
scsi events) with a certain overlap, so this really could do with a
cleanup.
d) leverage these events to handle path-up/path-down events
in-kernel
e) move the I/O redirection logic out of device-mapper proper
and use blk-mq to redirect I/O. This is still a bit of
hand-waving, and definitely would need discussion to figure
out if and how it can be achieved.
This is basically the same topic Mike Snitzer proposed, but
coming from a different angle.
Another (adjacent) topic is multipath performance with blk-mq.
As I said, I've been looking at nvme multipathing support and
initial measurements show huge contention on the multipath lock
which really defeats the entire point of blk-mq...
I have yet to report this as my work is still in progress. I'm not sure
if it's a topic on it's own but I'd love to talk about that as well...
Oh, most definitely. There are some areas in blk-mq which need to be
covered / implemented before we can even think of that (dynamic
queue reconfiguration and disabled queue handling being the most
prominent).
_And_ we have the problem of queue mapping (one queue per ITL nexus?
one queue per hardware queue per ITL nexus?) which might quickly
lead to a queue number explosion if we've not careful.
Cheers,
Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@xxxxxxx +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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