When looking at the port-level statistics, high values on ports used for messages to nodes in the same system are much more concerning than those on ports that are used for messages to nodes in other systems. 

GLYPH<SM590000> Back-end Write Response Time 

This value is the average response time in milliseconds for write operations to the back-end storage. This time might include several physical I/O operations, depending on the type of RAID architecture. 

Poor back-end performances on secondary system is a frequent cause of 1920 errors, while it is not so common for primary systems. Exact values to watch out for depend on the storage technology, but usually the response time should be less than 50 ms. A longer response time can indicate that the storage controller is overloaded. If the response time for a specific storage controller is outside of its specified operating range, investigate for the same reason. 

## Focus areas for 1920 errors 

The causes of 1920 errors might be numerous. To fully understand the underlying reasons for posting this error, consider the following components that are related to the remote copy relationship: 

GLYPH<SM590000> The intersystem connectivity network 

GLYPH<SM590000> Primary storage and remote storage 

GLYPH<SM590000> IBM FS9100 node canisters 

GLYPH<SM590000> Storage area network 

## Data collection for diagnostic purposes 

A successful diagnosis depends on the collection of the following data at both systems: 

GLYPH<SM590000> The snap  command with livedump  (triggered at the point of failure) 

GLYPH<SM590000> I/O Stats running (if possible) 

GLYPH<SM590000> IBM Spectrum Control performance statistics data (if possible) 

GLYPH<SM590000> The following information and logs from other components: 

– Intersystem network and switch details: 

GLYPH<129> Technology 

GLYPH<129> Bandwidth 

GLYPH<129> Typical measured latency on the Intersystem network 

GLYPH<129> Distance on all links (which can take multiple paths for redundancy) 

GLYPH<129> Whether trunking is enabled 

GLYPH<129> How the link interfaces with the two SANs 

GLYPH<129> Whether compression is enabled on the link 

GLYPH<129> Whether the link dedicated or shared; if so, the resource and amount of those resources they use 

GLYPH<129> Switch Write Acceleration to check with IBM for compatibility or known limitations 

GLYPH<129> Switch Compression, which should be tr ansparent but complicates the ability to predict bandwidth 

– Storage and application: 

GLYPH<129> Specific workloads at the time of 1920 errors, which might not be relevant, depending upon the occurrence of the 1920 errors and the volumes that are involved 

