NFC Session Limits and
Timeouts
NBDSSL employs the network file copy (NFC)
protocol.
Table 1
shows limits on the number of connections for various host types. These are
host limits, not per process limits. Additionally vCenter Server imposes a
limit of 52 connections.
VixDiskLib_Open()
uses one
connection for every virtual disk that it accesses on an ESXi host. Clone with
VixDiskLib_Clone()
also requires
a connection. It is not possible to share a connection across physical disks.
These NFC session limits do not apply to SAN or HotAdd transport.
Host Platform
| When Connecting
| Limits You To About
|
|---|---|---|
vSphere 3.5
| to an ESX host
| 9 connections directly, 27
connections through vCenter Server
|
vSphere 4.x
| to an ESXi host
| 11 connections directly, 23
connections through vCenter Server
|
vSphere 5.x
| to an ESXi host or vCenter
Server
| Limited by a transfer
buffer for all NFC connections, enforced by the host; the sum of all NFC
connection buffers to an ESXi host cannot exceed 32MB.
52 connections through vCenter Server,
including the above per-host limit.
|
vSphere 6.x
| to an ESXi host or vCenter Server
| The sum of all NFC connection buffers cannot
exceed 48MB by default. ESXi
hostd limit can be
changed with
config.xml but
vCenter is not configurable.
With 1MB buffers per I/O
stream, 48 requests can be concurrent.
|
The
InitEx
configuration file parameter
vixDiskLib.nfc.ReadTimeoutMs
indicates the preferred
connection timeout for backup and restore operations, and has the effect of
influencing TCP keep-alive time when no reads occur, which they do not during
prolonged restores. If the
ReadTimeoutMs
value is set higher than the TCP
keep-alive time on the network, then a connection timeout could occur.
For example, if the
configuration file sets
ReadTimeoutMs
flag to 3600000 milliseconds (60 minutes)
instead of accepting the default (varies from 6 to 45 seconds) then a TCP
keep-alive packet transmits only after 60 minutes. If restoring a virtual disk
takes longer, leaving network read activity idle for 60 minutes, the connection
could break so subsequent restores fail.