VDDK restore may fail with connection time out error if "vixDiskLib.nfc.ReadTimeoutMs" flag is set to a higher value

Products

• VMware vSphere ESXi

Issue/Introduction

Symptoms:

When restoring multiple files using NBD or NBDSSL transport mode, the virtual disk library could fail with a connection time-out if the vixDiskLib.nfc.ReadTimeoutMs value is set to a higher than default value.

Environment

VMware Virtual Disk Development 1.2

Cause

When a restore application opens all disks of a virtual machine at once, rather than one at time, then trying to restore them in succession, a TCP timeout could occur if restoring any virtual disk takes a long time.<?xml:namespace prefix = "o" />

The VDDK configuration file parameter vixDiskLib.nfc.ReadTimeoutMs is designed to indicate the preferred timeout for backup operations, but it has the side effect of influencing TCP keep-alive time when no reads occur, which they do not during a prolonged restore. If the ReadTimeoutMs value is set higher than the TCP keep-alive time on the network,than a connection time-out occurs.

For example, if the configuration file sets ReadTimeoutMs flag to 3600000 milliseconds (60 minutes) instead of accepting the default 6000 milliseconds (6 seconds in ESX 5.5) then a TCP keep-alive packet transmits only after 60 minutes. If restoring a virtual disk takes longer than that, leaving network read activity idle for 60 minutes, the connection breaks. Therefore, subsequent virtual disk restores fail.

Resolution

VMware recommends that the backup application should open the disk only when the application is ready to read/write the disk.

To work around this issue, set the ReadTimeoutMs value to a lower value than the prevailing TCP keep-alive time on your network.

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Additional Information

Impact/Risks: