6.7

If the transactions are large and multiple servers are sending data through a single switch port, an ability
to buffer can be exceeded. In this case, the switch drops the data it cannot send, and the storage system
must request a retransmission of the dropped packet. For example, if an Ethernet switch can buffer 32
KB, but the server sends 256 KB to the storage device, some of the data is dropped.
Most managed switches provide information on dropped packets, similar to the following:
*: interface is up
IHQ: pkts in input hold queue IQD: pkts dropped from input queue
OHQ: pkts in output hold queue OQD: pkts dropped from output queue
RXBS: rx rate (bits/sec) RXPS: rx rate (pkts/sec)
TXBS: tx rate (bits/sec) TXPS: tx rate (pkts/sec)
TRTL: throttle count
Table 13‑1. Sample Switch Information
Interface IHQ IQD OHQ OQD RXBS RXPS TXBS TXPS TRTL
*
GigabitEth
ernet0/1
3 9922 0 0 47630300
0
62273 47784000
0
63677 0
In this example from a Cisco switch, the bandwidth used is 476303000 bits/second, which is less than
half of wire speed. The port is buffering incoming packets, but has dropped several packets. The final line
of this interface summary indicates that this port has already dropped almost 10,000 inbound packets in
the IQD column.
Configuration changes to avoid this problem involve making sure several input Ethernet links are not
funneled into one output link, resulting in an oversubscribed link. When several links transmitting near
capacity are switched to a smaller number of links, oversubscription becomes possible.
Generally, applications or systems that write much data to storage must avoid sharing Ethernet links to a
storage device. These types of applications perform best with multiple connections to storage devices.
Multiple Connections from Switch to Storage shows multiple connections from the switch to the storage.
Figure 13‑3. Multiple Connections from Switch to Storage
1 Gbit
1 Gbit
1 Gbit
1 Gbit
vSphere Storage
VMware, Inc. 123