Introduction of Unresponsive NFS mount points
In a recent incident, a HANA system experienced unexpected downtime, despite minimal system load and recent OS and HANA upgrades. The root cause was identified as unresponsive NFS mount points, even those not directly used by HANA. This article delves into the details of this issue, its impact on HANA performance, and the recommended solutions.
The Problem
HANA periodically polls mount points using the Linux `stat` command. When a mount point becomes unresponsive, the `stat` command hangs at the OS level, leading to a cascade of issues:
* **Nameserver Unresponsiveness:** The HANA nameserver, responsible for managing database connections and metadata, becomes unresponsive.
* **Database Unresponsiveness:** The HANA database itself becomes inaccessible, preventing SQL connections to the SYSTEMDB and tenant databases.
* **ABAP Short Dumps:** Numerous ABAP short dumps are generated as a result of the failed database connections.
**The Culprit: Unresponsive NFS Mount Points**
Even if a mount point is not directly used by HANA, its unavailability can still trigger the `stat` command hang and lead to the aforementioned issues. This behavior is particularly problematic in scenarios where NFS shares are powered down during off-peak hours to conserve energy.
SAP Note 3434285: A Lifesaver
Fortunately, SAP Note 3434285 provides valuable insights into this issue and offers potential solutions. The note highlights an improvement in HANA 2.00.079 and later versions, which aims to mitigate the impact of unresponsive mount points. However, as the incident described in this article demonstrates, the issue can still occur under certain circumstances.
Recommended Solutions
1. Avoid Powering Down NFS Shares:
* Continuous Availability: Keep NFS shares powered on continuously to ensure uninterrupted HANA operations.
* Optimized Power Management: Implement optimized power management strategies to reduce energy consumption without compromising HANA availability.
2. Implement Robust Monitoring:
* Monitor NFS Share Health: Regularly monitor the health and responsiveness of all NFS shares.
* Alert on Unresponsive Mount Points: Configure alerts to notify administrators immediately when mount points become unresponsive.
3. Upgrade to the Latest HANA Version:
* Leverage the Latest Improvements: Upgrade to the latest HANA version to benefit from the improvements in handling unresponsive mount points.
4. Apply SAP Notes:
* Stay Updated: Regularly apply SAP Notes to address known issues and vulnerabilities.
Conclusion
The unexpected downtime caused by unresponsive NFS mount points underscores the importance of careful system configuration and monitoring. By following the recommended solutions, organizations can minimize the risk of similar incidents and ensure the optimal performance of their HANA systems.
Reference:
3544143 – HANA Database Unavailability Caused by Unresponsive Mount Points
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