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Clearly any reduction in traffic will only occur for inactive servers and won t do anything for servers that are used on a 24x7 basis, but it is an indication of how Microsoft has tweaked log replication to be more efficient based on the experience gained from Exchange 2007.
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Block replication
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Block replication is a new feature introduced in Exchange 2010 SP1 to minimize the delay between a transaction occurring in an active database and when it is copied and replayed to all of the database copies. The idea is that speeding up replication reduces the potential exposure to data loss in the case of failure caused when a transaction log is corrupted between the times that it is generated and copied. In this respect, a transaction log can be considered a potential single point of failure that Exchange needed to address to achieve true high availability. Exchange 2010 depends on the replication and replay of complete transaction logs to keep database copies updated. The current transaction log file is locked and the data that it holds are unavailable when it is in use. Exchange 2010 SP1 still uses transaction log replication but the log copier component includes some new code that monitors the state of replication to understand when it is safe enough to begin replication at the block level as data are flushed to disk. In this context, safe means that file-level replication is proceeding without problems and the database copies are all healthy. The following approach is taken by Exchange 2010 SP1 mailbox servers:
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All replication begins in file mode. When a server comes online, it contacts the servers that host the active copies of any of its databases to request replication of any log generations that it has not yet seen. The servers that host the active databases respond with copies of any outstanding logs. The server replays the logs and brings its database copies up to date. Once a database copy is current (in other words, the next transaction log it needs is the current transaction log), Exchange can switch over to block mode. In block mode, as transactions are written into the ESE log buffer (the 1 MB cache used to accumulate transactions to be written into a transaction log), the log copier copies the data in parallel to all of the servers that hold a database copy. The data are written into a similar log buffer on these servers. When the log buffer on the receiving servers is full, the complete buffer is written out as a transaction log. The new log is inspected to ensure that it is not corrupt, and if this check passes, the log is inserted into the log stream to be replayed into the passive database.
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Transaction log replay: The foundation for DAG replication
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Replication continues in block mode until a replay queue of four logs accumulates on a server. At this point, Exchange automatically switches back into file mode and replication continues in this mode until the replay queue is cleared and the next log required is once again the current transaction log. Exchange then transfers back into block mode.
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Because log file replication continues and the logs are verified both on the sending and receiving servers (a verification failure causes a log rewrite), the previous potential for failure caused by a corrupt transaction log is removed .
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An individual transaction might fit into a single 32 KB database page. New data obviously reach the servers that host database copies faster if the log copier only has to wait for and then copy a 32 KB page instead of a complete 1 MB transaction log. The case is less obvious for transactions generated by large multimegabyte messages as these require the replication of a number of transaction logs. However, sufficient small transactions fit into single pages to make block replication very attractive in terms of its ability to dramatically reduce overall replication latency. Block replication also changes the activation process when Exchange has to bring a passive database copy online. If a copy was using block replication when the failure occurred, Exchange uses whatever partial log content is available to create a complete transaction log that it uses to bring the passive copy up to date as it is activated. This step ensures that the newly activated database will have all available data in it when it is brought online. Later on, when the failed server comes back online, the incremental reseeding code is able to resolve any divergence that exists between the data in the fragment used to update the activated database and the data that it might have received had replication been complete. The normal resolution to any divergence is a request to recopy the complete log generation represented by the fragment that arrived from the database that has all the information. Once the missing log is distributed, it can be replayed into the other database copies to make sure that all copies contain the same information.
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