What is log sequence number ? How it is used in MySQL?
From mysqlperformanceblog


  Log Sequence Number (LSN) : Log Sequence Numbers correspond to given
  position in the log files and typically incremented for each log
  record. Innodb uses number of bytes ever written to the log files
  however it could be something different. LSNs are often extensively
  used in recovery check pointing and buffer management operations. When
  checkpoint (both fuzzy and not) happens you get something like “all
  changes up to LSN=X are now flushed to the data space” this means you
  can discard or archive logs for LSN earlier than that. When doing log
  recovery checking LSN in the log record can tell if you this change
  needs to be applied or it already was applied (when doing recovery you
  do not know which dirty pages were flushed from the buffer pool). 
  
  The LSN do not relate much to transactions – changes from different
  transactions are intermixed in the log files and many LSNs can
  correspond to changes from the same transaction.


For other references please have a look at

1 How to know the current log sequence number in 5.0.X.

2 How to check if Innodb log files are big enough