what is witness server used for
what is witness server used for? as far as i know, a witness server is a server outside of a dag that's achieve and maintain quorum when the dag has an even number of members. dags with an odd number of members do not use a witness server. Since that, once the number is odd, shall we configure witness server.
Could some one tell us why we need witness server?
Answers
- The file share witness is used to establish a majority node set. This is done by create a share on a server that gets a little file place into it automatically. The server hosting the cluster resource (which in the DAG I think is the Primary Activation Manager server) keeps an open file lock on this file. The other servers see this open file lock and interpret this as meaning another cluster node is online, healthy, and available.
A file share witness as you mentioned is used when the DAG contains an even number of servers withtin it. When you initially create the DAG you must specify the server and file location that will act as the file share witness regardless of how many servers are in the DAG (0 to start) to ensure that if you do add an even number of DAG members the FSW will be properly used.
You do not need a dedicated server for the FSWs and typically it is recommended to use a hub transport server in the primary datacenter. This is usually a safe thing to do as the Exchange team also manages the hub transport servers and the Exchange Trusted Subsytem will already be a member of the local administrators group and have the necessary permissions to create the file share. Some people put the FSW on clustered file servers or NetApps that are domain joined.
Brian Day: MCSA 2000/2003, CCNA, MCTS: Microsoft Exchange Server 2010 Configuration, Overall Exchange/AD Geek. Hi,
Good References:- http://technet.microsoft.com/en-us/library/dd298065(EXCHG.140).aspx
- http://www.simple-talk.com/content/print.aspx?article=808
Nitin Gupta (gupnit) | MVP - Exchange | http://www.nitingupta.in/blogs- Marked As Answer byJacky.Wu Thursday, November 05, 2009 5:43 AM
- Because an additional witness/referee is required when you have equal number of votes (with even numbers) and not when you don't have chance to get equal votes... :)
Amit Tank
MVP: Exchange Server | MCTS: Microsoft Exchange Server 2010, Configuration
MCITP: EMA | MCSA: M | Blog: http://ExchangeShare.WordPress.com- Marked As Answer byJacky.Wu Thursday, November 05, 2009 5:43 AM
When the number of members is odd, you cannot accidently have a split-brain situation where two datacenters think they have a majority node set and activate their databases in both datacenters, and you ensure one datacenter does in fact stay online.
Example...
1. You have two datacenters, Site-A and Site-B, and Site-A is the primary site users normally connect to.
2. Site-A and Site-B each have 3 servers that are members of a 6-node DAG
3. Network communication is lost between Site-A and Site-B
4. Site-A sees 3 nodes go offline in Site-B and only knows its own 3 nodes are online.
5. Site-B sees 3 nodes go offline in Site-A and only knows its own 3 nodes are online.
6. 6/3 = 50%, so you do *not* have a majority node set in either datacenter and the DAG cannot stay online.
7. The servers in Site-A dismount all databases.
8. The servers in Site-B dismount all databases.
9. All users are offline.
Now let's introduce a FSW.
01. You have two datacenters, Site-A and Site-B, and Site-A is the primary site users normally connect to.
02. Site-A and Site-B each have 3 servers that are members of a 6-node DAG
03. Site-A contains a FSW on a hub transport server, making up the 7th "member" of the DAG's majority node set.
04. Network communication is lost between Site-A and Site-B
05. Site-A sees 3 nodes go offline in Site-B and only knows its own 3 nodes are online plus the FSW.
06. Site-B sees 3 nodes go offline in Site-A and the FSW and only knows its own 3 nodes are online.
07. 4/7 in Site-A = 57% and you *do* have a majority node set in Site-A
08. 3/7 in Site-B = 42% and you do *not* have a majority node set in Site-B
09. The servers in Site-A keep all of their databases mounted.
10. The servers in Site-B dismount all databases until network communications are restored between datacenters.
11. All users remain online.
Brian Day: MCSA 2000/2003, CCNA, MCTS: Microsoft Exchange Server 2010 Configuration, Overall Exchange/AD Geek.- Marked As Answer byJacky.Wu Thursday, November 05, 2009 5:43 AM
All Replies
- The file share witness is used to establish a majority node set. This is done by create a share on a server that gets a little file place into it automatically. The server hosting the cluster resource (which in the DAG I think is the Primary Activation Manager server) keeps an open file lock on this file. The other servers see this open file lock and interpret this as meaning another cluster node is online, healthy, and available.
A file share witness as you mentioned is used when the DAG contains an even number of servers withtin it. When you initially create the DAG you must specify the server and file location that will act as the file share witness regardless of how many servers are in the DAG (0 to start) to ensure that if you do add an even number of DAG members the FSW will be properly used.
You do not need a dedicated server for the FSWs and typically it is recommended to use a hub transport server in the primary datacenter. This is usually a safe thing to do as the Exchange team also manages the hub transport servers and the Exchange Trusted Subsytem will already be a member of the local administrators group and have the necessary permissions to create the file share. Some people put the FSW on clustered file servers or NetApps that are domain joined.
Brian Day: MCSA 2000/2003, CCNA, MCTS: Microsoft Exchange Server 2010 Configuration, Overall Exchange/AD Geek. Hi,
Good References:- http://technet.microsoft.com/en-us/library/dd298065(EXCHG.140).aspx
- http://www.simple-talk.com/content/print.aspx?article=808
Nitin Gupta (gupnit) | MVP - Exchange | http://www.nitingupta.in/blogs- Marked As Answer byJacky.Wu Thursday, November 05, 2009 5:43 AM
- but could you tell me the reason why we dont use witness server when the nuber of memebers is odd.
- but could you tell me the reason why we dont use witness server when the nuber of memebers is odd.
- Because an additional witness/referee is required when you have equal number of votes (with even numbers) and not when you don't have chance to get equal votes... :)
Amit Tank
MVP: Exchange Server | MCTS: Microsoft Exchange Server 2010, Configuration
MCITP: EMA | MCSA: M | Blog: http://ExchangeShare.WordPress.com- Marked As Answer byJacky.Wu Thursday, November 05, 2009 5:43 AM
When the number of members is odd, you cannot accidently have a split-brain situation where two datacenters think they have a majority node set and activate their databases in both datacenters, and you ensure one datacenter does in fact stay online.
Example...
1. You have two datacenters, Site-A and Site-B, and Site-A is the primary site users normally connect to.
2. Site-A and Site-B each have 3 servers that are members of a 6-node DAG
3. Network communication is lost between Site-A and Site-B
4. Site-A sees 3 nodes go offline in Site-B and only knows its own 3 nodes are online.
5. Site-B sees 3 nodes go offline in Site-A and only knows its own 3 nodes are online.
6. 6/3 = 50%, so you do *not* have a majority node set in either datacenter and the DAG cannot stay online.
7. The servers in Site-A dismount all databases.
8. The servers in Site-B dismount all databases.
9. All users are offline.
Now let's introduce a FSW.
01. You have two datacenters, Site-A and Site-B, and Site-A is the primary site users normally connect to.
02. Site-A and Site-B each have 3 servers that are members of a 6-node DAG
03. Site-A contains a FSW on a hub transport server, making up the 7th "member" of the DAG's majority node set.
04. Network communication is lost between Site-A and Site-B
05. Site-A sees 3 nodes go offline in Site-B and only knows its own 3 nodes are online plus the FSW.
06. Site-B sees 3 nodes go offline in Site-A and the FSW and only knows its own 3 nodes are online.
07. 4/7 in Site-A = 57% and you *do* have a majority node set in Site-A
08. 3/7 in Site-B = 42% and you do *not* have a majority node set in Site-B
09. The servers in Site-A keep all of their databases mounted.
10. The servers in Site-B dismount all databases until network communications are restored between datacenters.
11. All users remain online.
Brian Day: MCSA 2000/2003, CCNA, MCTS: Microsoft Exchange Server 2010 Configuration, Overall Exchange/AD Geek.- Marked As Answer byJacky.Wu Thursday, November 05, 2009 5:43 AM
