6.0.1

When writing data to storage, multiple systems or virtual machines might aempt to ll their links. As
Dropped Packets shows, when this happens, the switch between the systems and the storage system has to
drop data. This happens because, while it has a single connection to the storage device, it has more trac to
send to the storage system than a single link can carry. In this case, the switch drops network packets
because the amount of data it can transmit is limited by the speed of the link between it and the storage
system.
Figure 12‑2. Dropped Packets
1 Gbit
1 Gbit
1 Gbit
dropped packets
Recovering from dropped network packets results in large performance degradation. In addition to time
spent determining that data was dropped, the retransmission uses network bandwidth that could otherwise
be used for current transactions.
iSCSI trac is carried on the network by the Transmission Control Protocol (TCP). TCP is a reliable
transmission protocol that ensures that dropped packets are retried and eventually reach their destination.
TCP is designed to recover from dropped packets and retransmits them quickly and seamlessly. However,
when the switch discards packets with any regularity, network throughput suers signicantly. The
network becomes congested with requests to resend data and with the resent packets, and less data is
actually transferred than in a network without congestion.
Most Ethernet switches can buer, or store, data and give every device aempting to send data an equal
chance to get to the destination. This ability to buer some transmissions, combined with many systems
limiting the number of outstanding commands, allows small bursts from several systems to be sent to a
storage system in turn.
If the transactions are large and multiple servers are trying to send data through a single switch port, a
switch's ability to buer one request while another is transmied can be exceeded. In this case, the switch
drops the data it cannot send, and the storage system must request retransmission of the dropped packet.
For example, if an Ethernet switch can buer 32KB on an input port, but the server connected to it thinks it
can send 256KB 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 12‑1. Sample Switch Information
Interface IHQ IQD OHQ OQD RXBS RXPS TXBS TXPS TRTL
*
GigabitEt
hernet0/1
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. In spite of this, the port is buering incoming packets and has dropped quite a few packets. The
nal line of this interface summary indicates that this port has already dropped almost 10,000 inbound
packets in the IQD column.
vSphere Storage
118 VMware, Inc.