[HDFS-10201] Implement undo log in parity datanode for hflush operations

• Type: Sub-task
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: erasure-coding
• Assignee: Rui Gao
• Reporter: Mingliang Liu
• Created: 23/Mar/16 18:00
• Updated: 01/Nov/16 23:54

Description

According to the current design doc for hflush support in erasure coding (see HDFS-7661), the parity datanode (DN) needs an undo log for flush operations. After hflush/hsync, the last cell will be overwritten when 1) the current strip is full, 2) the file is closed, 3) or the hflush/hsync is called again for the current non-full stripe. To serve new reader client and to tolerate failures between successful hflush/hsync and overwrite operation, the parity DN should preserve the old cell in the undo log before overwriting it.

As parities correspond to block group (BG) length and parity data of different BG length may have the same block length, the undo log should also save the respective block group (BG) length information for the flushed data.

This jira is to track the effort of designing and implementing an undo log in parity DN to support hflush/hsync operations.