1 Log File Can Contain More Records After Performing Backup Database Statement, 2 Why Can't Insert Data When Is Log Is Not Full

question 1:

i found that database log file can contain more records after performing backup database statement.

for example:

i create a database and limit the log file to 2mb. then i create a table and insert data.

If i backup the database before i insert data , the database file can contain 192 records unitl the log file is full.


If i don't perform the 'backup database' statement.
The 'dbcc sqlperf(logspace)' indicate the utilization ratio is less than 40% after inserting 192 records

why?

I list my code:



Code Snippet
create database db_test
on primary
(
name=db_test,
filename='C:Program FilesMicrosoft SQL ServerMSSQL.1MSSQLDatadb_test.mdf'
)
log on
(
name=db_test_log,
filename='C:Program FilesMicrosoft SQL ServerMSSQL.1MSSQLDatadb_test_log.ldf',
maxsize=2mb
)
go
backup database db_test to disk='db_test.bak' --- if i don't execute this line, log file can contain a lot of record
go
create table db_test..table1(col char(8000))
--insert data to fill up the database log
declare @n int
set @n=0
while @n<192
begin
insert into db_test..table1 values(replicate('a',8000))
set @n=@n+1
end








question 2:
i create a database and limit the log file to 2mb. Then i create a table and insert data in an endless loop.

After the inserting operation executing for a while, the 9002 error occurs, indicate the log file for the database is full.
But the 'dbcc sqlperf(logspace)' command indicate the unilization ratio is low, and log_reuse_wait_desc in sys.database is 'CHECKPOINT'
And I can insert data , and i'm sure the state of log_use_wait_desc is 'CHECKPOINT'.


As i known, the checkpoint can't truncate log under full recovery model. Only the back log operation can truncate the transaction log.
So log is not full, why 9002 error is encounterd. and why the log_reuse_wait_desc return 'CHECKPOINT'?


I list my code:



Code Snippet
create database db_test
on primary
(
name=db_test,
filename='C:Program FilesMicrosoft SQL ServerMSSQL.1MSSQLDatadb_test.mdf'
)
log on
(
name=db_test_log,
filename='C:Program FilesMicrosoft SQL ServerMSSQL.1MSSQLDatadb_test_log.ldf',
maxsize=2mb
)
go
create table db_test..table1(col char(8000))

--insert data to fill up the database log
declare @n int
set @n=0
while @n<>-1
begin
insert into db_test..table1 values(replicate('a',8000))
end








any suggestions?


thanks in advance.