## 2.4.1.2 System Appears to Hang with no new Connections and no Response from the SAP HANA Database
In cases where logs cannot be written, all DML statement will fall into wait status. This can lead to a failure of opening new connections because the system internally executes DML statements during the process. Typically, a full log volume is the cause for this.
## Basic System Checks
In cases where the SAP HANA database is no longer responding to any requests it is advisable to follow a bottom up approach to quickly identify the root cause of the problem. The following system checks will eliminate common operating system related problems which often lead to system hanging situations:
Firstly, check the system resources at the operating system level, by using the TOP command:
Check the values for CPU and MEM to make sure that these resources are not exhausted.
Secondly, check the disk utilization by running the disk free command: df -h and verify that no volume has reached 100% utilization (see example below).
Check whether you are able to connect to the indexserver process locally by using hdbsql to run a test-query, for example:
```
SELECT CURRENT_TIMESTAMP FROM DUMMY;
```
If this is successful then check the network stability between the clients facing the issue and the SAP HANA database host.
## Possible Root Causes
Some common situations which cause a system to freeze are given here with remedial actions to take:
Root cause: Log volume full either caused by disk full or quota setting
## Required action:
1. Check for the DISKFULL event in the indexserver trace or
2. Run the disk free command: df -h in the OS shell (the -h option is more readable and displays the output in units of 1024). A result such as the following with a value of 100% will immediately identify the culprit as a full log area:
3. Check the quota setting in the file system.
4. If you have verified that the log volume is full then follow the procedure described in SAP Note 1679938 Log Volume is full .
```
/dev/mapper/vg_hana-log       102G   102G   0G   100%  /hana/log
```
You can find additional information in SAP Note 2083715 - Analyzing log volume full situations .
Root cause: Log volume full caused by failure of log backup
## Required action:
1. Check backup.log (located at /usr/sap/&lt;SID>=/HDB&lt;Instance#>=/&lt;Host>=/trace ) to see whether it includes ERROR in log backup. Check m_BACKUP_CATALOG, m_LOG_SEGMENTS.
2. If log backup uses backint, please check backint.log (located at /usr/sap/&lt;SID>=/HDB&lt;Instance#>=/ &lt;Host>=/trace ) to see whether it includes ERROR information, and contact backint vendor support.
## Admission Control
Note that if statements are rejected with SQL Error 1038 this is related to the Admission Control feature which is designed to prevent the system from being overloaded by defining thresholds and either queuing or rejecting statements when necessary. You can check whether admission control is preventing new sessions from being established due to low system resources by querying the monitoring view m_ADMISSION_CONTROL_EVENTS:
```
SELECT * FROM M_ADMISSION_CONTROL_EVENTS;
```
If many events are raised during the problematic time-period you should investigate which SAP HANA resources were exhausted making it necessary for admission control to intervene. For more information about Admission Control see the KBA 2222250 FAQ: SAP HANA Workload Management
Before SAP HANA 2 SPS05 Revision 056 there is a known coding issue where the Admission Control feature could generate the SQL Error 1038 when there is no overloading of resources. For more information about this issue see the KBA 3061965 - Unexpected Error "rejected as server is temporarily overloaded" .
## Configuration Parameters for Admission Control
You should apply the recommended settings for all admission control configuration parameters located under indexserver.ini [admission_control] . Y ou can check the parameter settings using the script 'HANA_Configuration_Parameters_2.00.040+' from the SAP Note 1969700 SQL Statement Collection for SAP HANA . The example output below from this script shows a discrepancy for the setting queue_memory_threshold which is set to a value of 90 when on a SAP HANA Database with Revision &lt;= 2.00.053 it should be zero. The explanation for this setting is to avoid queuing in case of high memory consumption that is often harmless (e.g. caused by a large page cache). The latest information about recommended configuration parameter settings is maintained in SAP note 2600030 Parameter Recommendations in SAP HANA Environments .
Where Admission Control is functioning correctly the root cause for the Admission Control event(s) is often high CPU, this can be confirmed for current Admission Control events using the script 'HANA_Workload_AdmissionControlEvents' or for historic Admission Control events using the script 'HANA_LoadHistory_Services' from the SAP Note 1969700 SQL Statement Collection for SAP HANA .
For troubleshooting steps for high CPU analysis please see Analysis of CPU Related Issues .
## Related Information