[IMPALA-3680] Reset the file offset after failed cache reads

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Impala 2.6.0
• Fix Version/s: Impala 2.7.0, Impala 2.6.2
• Component/s: Backend
• Target Version: Impala 2.7.0, Impala 2.6.2
• Assignee: Bharath Vissapragada
• Reporter: Bharath Vissapragada
• Created: 06/Jun/16 11:19
• Updated: 08/Sep/16 16:27
• Resolved: 01/Jul/16 07:42

Description

Currently we don't reset the file read offset if ZCR fails. Due to this, when we switch to the normal read path, we hit the eosr of the scan-range even before reading the expected data length. This results in re-issuing the whole set of scan ranges and hence a severe
performance hit.

This has been observed while debugging IMPALA-3679, where zero-copy reads don't work with encryption zones. However this situation can apply to any general case where the ZCRs fail.