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Part V: Enterprise Data Management
it provides full data safety. The sequence of events when you choose safety full is
described here and shown in Figure 27-1.- The client submits a transaction to the principal database. The principal server
writes the transaction log records to the transaction log buffer. - The transaction log buffer is written to disk (also referred to as hardening of the
log) and simultaneously transfers the transaction log records from the buffer to
the mirror server. The principal server waits for a confi rmation from the mirror.
Because the principal waits for an acknowledgment from the mirror, the application
response time increases and transaction throughput slightly reduces. The actual
performance impact depends on various factors such as your network latency, disk
subsystem, application, and more.
There are no specifi c restrictions on the network for database mirroring, but the network connection between the
servers is critical. The process to determine a failover in synchronous database mirroring is based on the network
connection. If a problem exists with the network, mirroring fails over or denies access to the database because of the
quorum requirement. Although mirroring works as designed, this behavior may come as a surprise to new database
mirroring users because with a regular stand-alone server, the database is still available. It is recommended to have
a dedicated network of high quality and high bandwidth. As a rule, the network bandwidth should be three times the
maximum log generation rate.- The mirror server writes the transaction log records to the transaction log buffer.
The transaction log buffer is then written to disk. - The mirror server acknowledges that the transaction has been written.
- The commit is reported to the client.
If you choose safety full and have a witness server in the database mirroring confi gura-
tion, you can achieve automatic failover. This means if the principal database has a failure
or is unavailable, the witness server and the mirror server form a quorum, and the mirror
server performs an automatic failover. The mirror server becomes the new principal server
and recovers the database and starts servicing the clients. This operating mode is also
referred to as high-safety mode with automatic failover.If you choose safety full and you do not have a witness server in the database mirror-
ing confi guration, you cannot achieve automatic failover because if the principal database
fails, the mirror server cannot form a quorum because there is no witness server. In this
confi guration, you need to perform a manual failover. This operating mode is also referred
to as high-safety mode without automatic failover.c27.indd 690c27.indd 690 7/31/2012 9:50:24 AM7/31/2012 9:50:24 AM
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