Committed and uncommitted data in Log Backup in SQL

Transaction log backup captures the transaction log which contains a record of all committed or uncommitted transactions.

In full recovery model, some time log file getting full that and first we will check log bak taken r not the we ill take log backup then it will clear all committed transaction from log file this will happen if have doubt create on database do some bulk(select into or insert into) trans and check log file size and take t log back then compare sizes.

Transaction log is a physical file in which SQL server stores the details of all transactions and data modifications performed on the database.In the event of of disaster, that causes SQL server to shutdown unexpectedly (Power failure/hardware failure), the transaction log is used to bring the database in a consistent state while restarting the server.On restarting the server, database goes through the recovery process.During this recovery process , the transaction log is used to make sure that all committed transactions are written to respective data pages (rolled forward) and revert the uncommitted transaction that were written to data pages.

Logically transaction log is a set of log records.Each records is identified by a log sequence number (LSN). The new log records is always written at the logical end of log file with a LSN which is greater than the previous one.Each LSN will be associated with a transaction id , which can be used to group the log records of a transaction. As log file store the log records in the sequential order as it happens, It is not necessary that, log records of a transaction are always available in sequence in the log file.Each log records will have the LSN of previous log as a backward pointer and that will help in rollback of transaction.

Transaction log will store separate log entries for each operation.For example, while inserting a record into a table, transaction log will store separate log entry for inserting into clustered index and other non clustered index. In the same way, if a single update statement is updating 10 records, transaction log will capture 10 separate log entries.For data modification, transaction log store either the logical operation performed or the before and after image of the record.

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