Information Technology Reference
In-Depth Information
weighted algorithm, nor is it responsive to queue depth, but it is a signii cant improvement.
In prior ESXi versions, there was no way to load balance a LUN, and customers needed to
statically distribute LUNs across paths, which was a poor proxy for true load balancing.
Which PSP Is Right If You're Using ALUA?
What do you do if your array can be confi gured to use ALUA—and therefore could use the Fixed,
MRU, or Round Robin policy? See the section “Understanding Midrange and External Enterprise
Storage Array Design” for information on ALUA.
h e Fixed and MRU path failover policies deliver failover only and work fi ne with active-active and
active-passive designs, regardless of whether ALUA is used. Of course, they both drive workloads
down a single path. Ensure that you manually select active I/O paths that are the “good” ports,
which are the ones where the port is on the storage processor owning the LUN. You don't want to
select the “bad” ports, which are the higher-latency, lower-throughput ones that transit the internal
interconnect to get to the LUN.
h e out-of-the-box load-balancing policy in vSphere (Round Robin) doesn't use the non-optimized
paths (though they are noted as active in the vSphere Web Client). h ird-party multipathing plug-
ins that are aware of the diff erence between the asymmetrical path choices can optimize an ALUA
confi guration.
Perform the following steps to see what SATP (and PSP) is being used for a given LUN in the
vSphere Web Client:
1.
In the vSphere Web Client, navigate to the Hosts And Clusters view.
2.
Select a host from the list on the left; then select the Manage tab on the right.
3.
Click the Storage subsection.
4.
Finally, click the Storage Devices selection on the left.
This opens the Storage Devices area. When a LUN or disk is selected from the list, an
SATP will be listed near the bottom, as shown in Figure 6.21.
In this example, the array is an EMC VNX and the generic VMW_SATP_ALUA_CX is selected. The
default PSP is Round Robin (VMware). A change in the PSP takes place
immediately
when you
change it. There is no coni rmation. Note that the PSP is coni gurable on a LUN-by-LUN basis.
What Is All the Stuff in the Storage Device Details List?
In the runtime name, the
C
is the channel identifi er, the
T
is the target identifi er, and the
L
is
the LUN.
And that long text string starting with
naa
? h is is the Network Address Authority ID, which is a
unique identifi er for the target and a LUN. h is ID is guaranteed to be persistent through reboots
and is used throughout vSphere.
