SQL Server Transaction logarchitecture and Virtual log filesTransaction log is reusable circular file which captures transaction details to recover database in disaster. Sql transaction Log file is divided into number of virtual log files (VLF) to handle active and inactive transaction in the database. Sql server transaction log writes entries sequentially into the virtual log files one by one. When one file is full, SQL move to next virtual log file (VLFs) till it reaches to last one. After reaching at the end, it will go back to first virtual log file which in inactive state and start using again.Virtual Log file can be either in active or inactive statesWhat is Active state in Virtual Log ?The log records that are currently used by sql server database engine, it can contain more than one virtual log files that we call it as active virtual log file. The log records may part of active transactions, replication, mirroring or log shipping. Active virtual log may be part of transaction which is already completed but not yet written back to the data file.What is Inactive state in Virtual Log ?The log records that are written back to data file and also not in use by database engine is known as inactive virtual log file. The log records may not be part of active transactions, replication, mirroring or log shipping. Whenever log backed up in full and bulk logged recovery model, virtual log file becomes inactive.How to reuse transaction log?Transaction log is cyclic process of writing log record into virtual log file by SQL Server, whenever active virtual log file filled up with data it moves to next inactive virtual log file. In short it needs inactive virtual log file for writing log records. If all virtual log files are filled up with active data then it will grow further to create more virtual log file. If it cannot grow then it will throw error saying transaction log for database is full and transaction will fail.To find the reason why transaction log cannot be reuse, execute the below querySelect log_reuse_wait_desc from sys.databases where name = yourdbnameTo reuse virtual log files, we have to truncate logs. After truncation it will release space and mark as inactive virtual log which is ready to use by SQL.Lets understand with the SQL Server Transaction log architecture diagram, assuming below is the transaction log file which divided into 5 virtual log files (v1, v2, v3, v4 and v5). In this V1 and V2 are in inactive state and V3, V4 and V5 are in active state which associated with active transaction in database.

In the above diagram, LB indicates last transaction log backup for the log records and CKP indicates the last checkpoint for associated log records.

After number of data modification in log records, SQL moves to the last virtual log file (V5). Now it will look for inactive virtual log file for writing more log records so it will go back to first inactive virtual log file (V1) and start writing log records in V1 and V2. Here we can see that V3, V4 and V5 are in still active state. Now to mark them as inactive, log backup should be run that will release some space for SQL to reuse them.

Otherwise if log records keep on growing and finished with writing in V1 and V2 virtual log files then it will throw error saying transaction log for database is full.

Understanding the VLF(Virtual Log File)

A database can have one or more log file. In general there will be only one log file as there is no performance improvement by having multiple log file. SQL server uses the transaction log in sequential manner.As the data file divided into pages,log files are divided into virtual log file(VLF).The size of the VLFs in a log file may not be in equal size. SQL server decide the size and number of VLF in a log file based on the size of the log file growth as given below.

Growth upto 64 MB = 4 VLFFrom 64 MB to 1 GB = 8 VLFLarger than 1 GB = 16 VLF

Let us create a database with 64 MB initial log size and later increase it to 1 GB. As per above calculation the log file should have 12 VLFs. 4 VLF based on initial size and 8 VLF due to changing the log size to 1 GB.

USE MASTER;GOCREATE DATABASEMydbON( NAME=MyDb_dat,FILENAME='D:\MyDb\Mydb.mdf', SIZE=10MB,MAXSIZE=3072MB, FILEGROWTH=5MB)LOGON(NAME=MyDb_log,FILENAME='D:\MyDb\MyDB.ldf',SIZE=64MB,MAXSIZE=2048MB,FILEGROWTH=5MB) ;GO

ALTER DATABASEMydbMODIFYFILE(NAME=MyDb_Log,FILENAME='D:\MyDb\MyDB.ldf',SIZE=1024MB)

Now Let us see how many VLF got created. To find out the number of VLF in database log file, we can make use of DBCC Loginfo.DBCCloginfo('mydb')

The output is given below.

There are 12 records in the output each represent a VLF.Let us try to understand the result

FileId:This is the file id of the log file and will be same for all 12 records as we have only one log file.If we have multiple log file , we can multiple numbers here

FileSize:This is the size of the VLF. If you look into the first four, have same size except the fourth one. This because first 8KB of the log file is used for file header. If you add filesize value of first four records along with 8192(8KB) , you will get 64MB which is the initial size of the log file.16711680+16711680+16711680+16965632 =67100672+8192 =67108864bytes =64MBIn the same if you add the last 8 records it will account the 960 MB (1024-64) , the growth happened due to the alter statement.

StartOffSet:This values is also in bytes, and is the sort column of the output. The first VLF alwasy start from 8192, which is the number of bytes in a page.As mentioned above, the first 8KB is used for file header and will not store any log.

FSeqNo:The file sequence number indicates the order of usage of the VLFs. The row with the highest FSeqNo value is the VLF where current log records are being written.FSeqNo values are not consistent. It will keep changing each time when VLF are getting reused. We will discuss more about this later in this post. A value of 0 in this column means that this VLF has never been used at all. That is the reason we have 0 for all records except one where it is currently logging.

Status:Status has two possible values : 0 and 2. A value of 2 means the VLF is not reusable and a value 0 means it can be reused.It will be more clear as we go further.

Parity:Parity has three possible values 0 ,64 and 128. If the VLF is not used yet, it will have a value 0 and will be set to 64 on first use.Every time a VLF is reused, the parity value is switched between 64 and 128.

CreateLSN:The value indicates when the VLF is created or to group the VLF based on the creation. A values 0 indicates, those VLFs are created as part of database creation. In our case first four records has a value 0 which indicate these VLFs are created as part of database creation with 64MB log size. The remaining 8 records has the same value. These VLF are created as part of our alter database statement to increase the size of the log file from 64 MB to 1024MB

The above output description is referred from Kalen Delaney BlogPost

Now our transaction log will looks like below