December 10, 2010 at 8:54 am
Hi guys,
I was just doing some tests with my two instance's of SQL Server 2008 R2 and 10 Mbps limited LAN. This is what I don't understand:
I setup synchronous database mirroring. Database log file is 25000 MB and 99% free before the test. I started to move some records and rebuild every index in database, that took 2 hours and 7 minutes. For the last hour in that test the network utilization was 99% used, before that the usage was up and down ...
Immediately after test ended (2 hours and 7 minutes) the network utilization went to 0%.
This is was I don't understand. After these actions database log file is 23% free which means that this test generated 19 GB of log records. How can 19 GB of log records be transferred to mirror instance if my network has 10 Mbps? 10 Mbps has 1.20 MB/s which means it would take than 4 hours to transfer all that data?
Kind regards
December 10, 2010 at 9:01 am
From Books Online (emphasis mine):
Database mirroring involves redoing every insert, update, and delete operation that occurs on the principal database onto the mirror database as quickly as possible. Redoing is accomplished by sending a stream of active transaction log records to the mirror server, which applies log records to the mirror database, in sequence, as quickly as possible. Unlike replication, which works at the logical level, database mirroring works at the level of the physical log record. Beginning in SQL Server 2008, the principal server compresses the stream of transaction log records before sending it to the mirror server. This log compression occurs in all mirroring sessions.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
December 10, 2010 at 9:04 am
sql2008 uses compression to ship log records to its mirrored dbs.
Johan
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December 10, 2010 at 9:15 am
Oh, yes, that explains it!
Btw, sorry if I didn't ask the question in a proper way.
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