Understanding Oracle Data-guard

-

 Data guard is a feature of Oracle High availability .

We have 2 types of dataguard 

1-> logical standby 

2- physical Standby 

in this section I will discuss about physical standby 

Architecture

when a user process connects to the primary server it will perform DML operations as part of this the modified information is recorded in the log buffer. From log buffer data will be updated in to ORL(online redo logs ) using LGWR as redo entries. 

The updated information / redo entries are read by the LNS(log network service) from redo buffer and passes through oracle net service and at the other side RFS process receives the redo data and applies on the SRL


LNS Supports 2 types of transfer modes 

Synchronous and Asynchronous transmission 

In synchronous

  • user commits a transaction creates a redo in redo buffer of SGA and it is being updated to ORL and waits for the confirmation from LNS as it reads the same data from redo and transmits to RFS to SRL
  •  RFS will send a write complete ACK to LNS and it is being notified to LGWR , and this is where user will get commit confirmation

Async:

 LGWR will continue to acknowledge commit success even if the bandwidth prevents the redo of previous transaction from being sent to the standby database immediately. If the LNS is unable to keep pace and the log buffer is recycled before the redo is sent to the standby, the LNS automatically transitions to reading and sending from the log file instead of the log buffer in the SGA. Once the LNS has caught up it then switches back to reading directly from the buffer in the SGA.


What is Automatic GAP resolution: 

It is a common scenario in DG where a LGWR will send the commit ack to the user irrespective of the redo transmission ACK from LNS by RFS and this could be due to network issue. In this case an archiver process will come into business as it frequently queries the standby control file to determine the last applied redo  and then transmits missing  archive files(offline redo logs) and RFS to apply to the SRL.  Once the GAP got reduced the LNS will come into business by reading the redo buffer and transmits to RFS to apply to SRL.

what is redo Apply:

Redo apply is basically a block by block physical replica of the primary database. it uses MRP to read records from SRLs into memory and apply change vectors to the standby database

It allows you to be able to use the standby database in a read-only fashion, Active Data Guard solves the read consistency problem in previous releases by the use of a "query" SCN. The media recovery process on the standby database advances the query SCN after all dependent changes in a transaction have been fully applied.


Protection Modes:

  • Maximum Performance 
  • Maximum Availability 
  • Maximum Protection

Maximum Performance :

This mode uses ASYNC redo transport so that the LGWR process never waits for acknowledgment from the standby database, This is the default mode.

Maximum Availability :

This mode uses  SYNC redo transport,  if standby server is unavailable the primary will wait the specified time in the NET_TIMEOUT parameter before giving up on the standby server and allowing the primary to continue to process. Once the connection has been re-established the primary will automatically resynchronize the standby database.

When the NET_TIMEOUT expires the LGWR process disconnects from the LNS process, acknowledges the commit and proceeds without the standby, processing continues until the current ORL is complete and the LGWR cycles into a new ORL, a new LNS process is started and an attempt to connect to the standby server is made, if it succeeds the new ORL is sent as normal, if not then LGWR disconnects again until the next log switch, the whole process keeps repeating at every log switch, hopefully the standby database will become available at some point in time.

Maximum Protection:

This  mode is data protection, even to the point that it will affect the primary database. This mode uses the SYNC redo transport and the primary will not issue a commit acknowledgment to the application unless it receives an acknowledgment from at least one standby database, basically the primary will stall and eventually abort preventing any unprotected commits from occurring.


  • Physical Standby - open read-only
  • Snapshot Standby - QA and Testing (convert database 'prod' to snapshot standby;)
  • Real Application Testing (RAT)
  • Active Data Guard




Comments

Post a Comment

Popular posts from this blog

Understanding Terraform

-
 Terraform is a tool used  for building, changing, and versioning infrastructure safely and efficiently. A Terraform can manage multiple cloud services using a provider plugin  Terraform work flow? Terraform work flow will be comprised of validate -> plan -> apply and finally destroy. what is the important file of terraform which records the actions of Terraform apply? For every Terraform execution the action will be recorded in a file called Terraform state file .  The terraform init command is used to initialise a working directory containing Terraform configuration files. During init, the configuration is searched for module blocks, and the source code for referenced modules is retrieved from the locations given in  their source arguments. The terraform plan command is used to create an execution plan. It will not modify things in infrastructure. The terraform apply command is used to apply the changes required to reach the desired state of the configur...
Read more

How to make CRS and ASM not to restart after server reboot

-
I have noticed that server rebooting is taking lot of time on a VM machine for every reboot, so I have decided to auto start of the crs and ASM service During this phase I have noticed an attribute called auto_start when I try to execute the below command crsctl stat res -p This AUTO_START attribute is there for several resources like ora.ASM ,ora.crs,ora.DATA_DG it has got 2 options restore(0) , never(2) and always (1) always (1)  Causes the resource to restart when the node restarts regardless of the resource’s state when the node stopped -restore (0)  Does not start the resource at restart time if it was in an offline state, such as STATE=OFFLINE, TARGET=OFFLINE, when the node stopped. The resource is restored to its state when the node went down. The resource is started only if it was online before and not otherwise. -never (2)  Oracle Clusterware never restarts the resource regardless of the resource’s state when the node is stopped. So I have dec...
Read more

How to repair ASM disk header

-
Image
What is ASM disk header corruption? ASM disk header corruption is a loss or damage to the disk header which contains the metadata essential for the operation and availability of an ASM disk group in Oracle. Imagine that you have an ASM disk corruption in any of the ASM diskgroups that have critical business data and the diskgroup has been configured as "EXTERNAL" redundancy, meaning that if one disk is inaccessible the whole diskgroup will be unavailable. Now what? Is your only option to restore it from backup by recreating the diskgroup or are there any other faster ways to recover the data? Here you need to find out whether it’s a complete loss of data in the ASM disk or is it just a header corruption. Hopefully for you, this is just a header corruption. Because, yes, there is a way out!  Below find the steps to restore from the ASM header corruption. Most of the data in the ASM disk header is of interest to that disk only. However, some information in the ASM disk header ...
Read more