Hi,
As we are using a very similar hardware and usage as Mark (Dell
poweredge hosts, Dell Equallogic SAN, iSCSI, and tons of LUNs for all
those VMs), I'm jumping into this thread.
Le 12/01/2017 à 16:29, Yaniv Kaul a écrit :
While it's a bit of a religious war on what is preferred with iSCSI -
network level bonding (LACP) or multipathing on the iSCSI level, I'm
on the multipathing side. The main reason is that you may end up
easily using just one of the paths in a bond - if your policy is not
set correct on how to distribute connections between the physical
links (remember that each connection sticks to a single physical link.
So it really depends on the hash policy and even then - not so sure).
With iSCSI multipathing you have more control - and it can also be
determined by queue depth, etc.
(In your example, if you have SRC A -> DST 1 and SRC B -> DST 1 (as
you seem to have), both connections may end up on the same physical NIC.)
If we reduce the number of storage domains, we reduce the number
of devices and therefore the number of LVM Physical volumes that
appear in Linux correct? At the moment each connection results in
a Linux device which has its own queue. We have some guests with
high IO loads on their device whilst others are low. All the
storage domain / datastore sizing guides we found seem to imply
it’s a trade-off between ease of management (i.e not having
millions of domains to manage), IO contention between guests on a
single large storage domain / datastore and possible wasted space
on storage domains. If you have further information on
recommendations, I am more than willing to change things as this
problem is making our environment somewhat unusable at the moment.
I have hosts that I can’t bring online and therefore reduced
resiliency in clusters. They used to work just fine but the
environment has grown over the last year and we also upgraded the
Ovirt version from 3.6 to 4.x. We certainly had other problems,
but host activation wasn’t one of them and it’s a problem that’s
driving me mad.
I would say that each path has its own device (and therefore its own
queue). So I'd argue that you may want to have (for example) 4 paths
to each LUN or perhaps more (8?). For example, with 2 NICs, each
connecting to two controllers, each controller having 2 NICs (so no
SPOF and nice number of paths).
Here, one key point I'm trying (to no avail) to discuss for years with
Redhat people, and either I did not understood, either I wasn't clear
enough, or Redhat people answered me they owned no Equallogic SAN to
test it, is :
My (and maybe many others) Equallogic SAN has two controllers, but is
publishing only *ONE* virtual ip address.
On one of our other EMC SAN, publishing *TWO* ip addresses, which can be
published in two different subnets, I fully understand the benefits and
working of multipathing (and even in the same subnet, our oVirt setup is
happily using multipath).
But on one of our oVirt setup using the Equallogic SAN, we have no
choice but point our hosts iSCSI interfaces to one single SAN ip, so no
multipath here.
At this point, we saw no other mean than using bonding mode 1 to reach
our SAN, which is terrible for storage experts.
To come back to Mark's story, we are still using 3.6.5 DCs and planning
to upgrade.
Reading all this is making me delay this step.
--
Nicolas ECARNOT
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