Change iSCSI Client Registry Setting

An iSCSI client on a network experiencing temporary performance issues may lose connection to its virtual resources, causing any applications relying on those resources to crash.

The iSCSI timeout registry settings on the iSCSI client can be changed to be more resistant to temporary network issues.

By default, the Microsoft initiator is able to sustain a network disconnect for up to 60 seconds. Should the network issues last for more than a minute, this can cause issues to the iSCSI client.

To prevent possible influence caused by temporary network outages, the Microsoft iSCSI Registry keys can be altered to extend the disk timeout. Keep in mind that you will need to reboot the machine after modifying the registry key, so it is best to make this registry change during a scheduled maintenance window.

Run regedit and locate the following key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E97B-xxxxxxxxxxxxxxxx}

This is SCSI and RAID controller class and details can be viewed on the right-hand panel. The key tree can be expanded to reveal additional folders such as 0000, 0001, etc., which are separate adapter instances. Find the one with DriverDesc of Microsoft iSCSI Initiator and navigate to the Parameters folder. On the right hand pane will be several DWORD values such as DelayBetweenReconnect and EnableNOPOut

Change the DWORD value named MaxRequestHoldTime from its default value of 60 to 300 (or from 3c to 12c if the value is in hexadecimal). This change will allow the iSCSI client and its hosted applications to withstand a network outage of up to five minutes.

Determine Replication Throughput Parameter

Two parameters most often used to adjust replication throughput:

Window Size

This is the transport window for the IPStor SAN/IP protocol. The window size is in packets (1500 bytes). The default value is 320, for a window of about 480KB. FalconStor suggests a setting of 1024 for a window of about 1.5 MB. The total window size of data will not actually be sent unless it is generated by outstanding replication commands. Therefore, it doesn't hurt to make this a little larger than necessary.

The following formula can be used to figure out the best possible parameters for window size:

Window-size=Line speed (T1, T3, and OC3)*Latency (millisecond)*1.2(Overhead latency)*2(roundtrip)/1.5

Based on the formula above and a series of tests, we recommend using the default setting for T1 line. It is more than enough to handle even 100ms latency.

For a T3 line, the default setting is also recommended for most cases. If the latency is greater than 50ms, the recommended window-size=1024.

For OC3 line, if the latency is greater than 50ms, the recommended setting will be window-size=4096. However if the latency is greater than 10ms and less than 50ms, the use of such setting may not achieve the best performance. The recommended setting in this case is window-size=640.

Replication-cmdsout

This parameter controls how many outstanding commands are sent by the source IPStor server during replication. The default value is 4. Each command is 64KB in size, so the default is 256 KB of outstanding data. Make sure to specify an appropriate value that keeps the channel full of data.

The following formula can be used to figure out the best possible parameters for replication-cmdsout:

Cmdsout= Line speed (T1, T3, and OC3)*Latency (millisecond)*1.2(Overhead latency)*2(roundtrip)/64

Based on the formula above and a series of tests, we recommend using the default setting for T1 line. It is more than enough to handle even 100ms latency.

For a T3 line, the default setting is also recommended for most cases. If the latency is greater than 50ms, the recommended cmdsout=24.

For OC3 line, if the latency is greater than 50ms, the recommended setting will be cmdsout=84. However if the latency is greater than 10ms and less than 50ms, the use of such setting may not achieve the best performance. The recommended setting in this case is cmdsout=12.

Check the actual window size and replication activity: cat /proc/krudp

The line speed used in the above formulas is MB/s not Mb/s. The latency is one way only. If using the round trip latency, drop the *2 in the formula.

The window size is changed at the server level in the IPStor Console. Right click on the IPStor server name and open the properties window. Under the performance tab, set the window size.

To change the cmdsout, pull down the help menu on the top line bar of the IPStor Console and click ABOUT. Hit the TAB key until the OK button is highlighted. Type sysvar , this will open the system variable window where the replication cmdsout parameter can be adjusted.

Port Required for Remote Monitoring using IPStor Console

TCP/11576 : Used for SAN Client and IPStor Console to IPStor Server management communication
TCP/11588 : Used for CCM Console for communication with IPStor Server
TCP/11762 : Used for IPStor Console and IPStor Server to SAN Client management communication