Database
Mirroring Enhancements in SQL Server 2008 from 2005
Database mirroring is
an alternative high-availability solution to failover clustering in SQL Server
Enterprise. Database mirroring supports automatic failover, but does not require
cluster-capable hardware, and can therefore provide a cost-effective
alternative to failover clustering.
This article is not focused
on to make you understand Database Mirroring Concept, rather this focuses onEnhancements,
which are being done in SQL Server 2008 for Database Mirroring
Database Mirroring Enhancements in SQL Server 2008 from 2005
·
Page-level
mirroring:
o If a page on the principle or mirror server is
corrupt, it is automatically replaced with corresponding copy on its partner
·
Automatic
Page Repair on Mirror Servers
o
If a page on the
principle or mirror server is corrupt, it is automatically replaced with the
corresponding copy on its partner
o
Some page types cannot
be automatically repaired:
§ File header pages
§ Database boot page
§ Allocation pages
o
I/O errors on the
principle server may be fixed during the mirroring session
o
I/O errors on the
mirror server require the mirroring session to be suspended
·
Compressed
Data flow
o
Data Flow between the
principle and mirror server is now compressed to improve performance
·
Manual
Failover
o
Manual failover no
longer require a database restart
·
Log
Performance
o
Write-ahead on the
incoming log stream on the mirror server
o
Improved use of
log-send buffers
o
Page read-ahead during
the undo phase after a failover
Database mirroring
enables you to maintain two copies of a database. One copy is the principal
server that client computers access. The other copy acts as a standby server.
In the case of a failure of the principal server, the client computers can use
the failover capability to connect to the standby computer with no loss of
data. Mirroring can therefore increase the availability of a database and
provide data protection. SQL Server 2008 offers several enhancements to the
database-mirroring environment, including the following:
·
Page-level
mirroring. This replaces
corrupt pages on one server with the same page on the partner server.
·
Compressed
data flow. This provides
improved performance and reduces the network bandwidth that database mirroring
uses.
·
Manual
failover. This no longer
requires a restart of the database
·
Log
performance. This has the
following improvements:
o
Write-ahead on the
incoming log stream on the mirror server. This writes the incoming log records
to disk asynchronously.
o
Improved use of
log-send buffers. If the most recently used log cache contains enough free
space for the current log records, they are appended to that log cache.
o
Page read-ahead during
the undo phase after a failover. The new mirror server sends read-ahead hints
to the principal server and the principal server puts those pages in its send
buffer. This process improves the speed of the undo phase.
Before SQL Server
2008, data restore could occur at the file level only. A corrupt page may
require a failover and then a restore of the file that contains the corrupted
data. This procedure is expensive due to the resources and time that are used,
and because more data is replaced than was actually corrupted.
SQL Server 2008
provides recovery at the page level. The database-mirroring environment
automatically replaces corrupt pages on one server with the same pages from the
partner server. The process does not require user intervention and does not
interrupt the availability of the server. By using automatic page repair, the
principal and mirror computers can recover from data page errors and from
errors that prevent reading a data page.
If a mirror server
finds a page with errors, it puts the mirroring session into the SUSPENDED
state, logs the error, and then requests a copy of the page from the principal
server. If the principal server can access the page, it sends a copy to the
mirror server, which replaces the page and resumes the mirroring session.
Otherwise, the mirroring session remains in a suspended state.
Automatic page repair
is only available in SQL Server Enterprise. However, if you have one mirror
running on Enterprise and one mirror on Standard, corrupt pages can be repaired
on the Enterprise instance, but not on the Standard instance
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