Why fragmentation transaction log is problematic

SQL Server transaction log files have an internal structure called the Virtual Log File or VLF. When the number of VLFs grow out of control due to autogrowth the log can become fragmented and cause delay.

Many processes that can be affected by the delay are: database and log backup, crash recovery.

There are more:

• Change data capture job
• Replication log reader.
• Database Mirroring

In the article the author demonstrated that whenever a transaction needs to grow up, it will allocate new VLFs:

http://www.mssqltips.com/sqlservertip/2107/performance-issue-with-large-number-of-virtual-log-files-in-sql-server-transaction-log/