Sql-server – tempdb log file usage huge by sleeping status sessions but not relased the space

As Max mentioned the alert probably fired just prior to the log growing/needing to grow. SCOM collects transaction log free space % although I am not sure at what threshold the alert will fire.

here is a quick example to show you what state tempdb is probably in when you get these alerts but no log file growth.

first create a database, set recovery to full, and back it up

    create database tlogspace
    on(name=tlogspace_dat,
        filename='c:\temp\tlogspace.mdf',
        size=4MB)
    log on (name=tlogspace_log,
        filename='c:\temp\tlogspace.ldf',
        size=1MB);
    go

    alter database tlogspace set recovery full;
    go
    backup database tlogspace to disk='nul';
    go

now switch to that database, create a table, and run DBCC sqlperf(logspace) to check the size oand free space in your log file.

use tlogspace
go

create table data(id int identity(1,1), col varchar(8000))
dbcc sqlperf(logspace)

On my system I have logfile size of 0.9921875 and log space used (%) of 48.4245. Now insert some data into the table and run DBCC sqlperf(logspace) again. On my system 45 rows inserted gave desired results(number of rows inserted may need to be adjusted).

insert into data(col)
select replicate('a',8000)
go 45 --may need to adjust number
dbcc sqlperf(logspace)

This time, the DBCC sqlperf output should show that log size is the same but log space used is just under 100%. In this case, SCOM would probably fire an alert that log space is low. There is no more activity going on to cause the log file to grow and (in this example) no tlog backup to free up used space. tempdb is in simple recovery, so your active transaction probably used up most of the available space and did not release it but there was not enough activity in tempdb to trigger log file growth thus causing the alert fire.

cleanup database when finished

use master
drop database tlogspace

Yes, SQL Server files do not shrink automatically. They remain the same size unless you explicitly shrink them, either through the SQL Server Management Studio or by using the DBCC SHRINKFILE command. While you can set files to autoshrink, this is widely advised against because shrinking/growing files is very resource intensive, especially when you're dealing with log files which don't have instant file initialization.

EDIT: In response to your edit: there is a difference between MAXSIZE and the size a file can get before running out of disk. You can set that in the Files section of the database properties, or with an ALTER DATABASE command.