[HBASE-23995] Snapshoting a splitting region results in corrupted snapshot

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 2.0.2
• Component/s: snapshots
• Assignee: Unassigned
• Reporter: Szabolcs Bukros
• Created: 16/Mar/20 12:44
• Updated: 26/Mar/20 17:15

Description

The problem seems to originate from the fact that while the region split itself runs in a lock, the compactions following it run in separate threads. Alternatively the use of space quota policies can prevent compaction after a split and leads to the same issue.

In both cases the resulting snapshot will keep the split status of the parent region, but do not keep the references to the daughter regions, because they (splitA, splitB qualifiers) are stored separately in the meta table and do not propagate with the snapshot.

This is important because the in the freshly cloned table CatalogJanitor will find the parent region, realizes it is in split state, but because it can not find the daughter region references (haven't propagated) assumes parent could be cleaned up and deletes it. The archived region used in the snaphost only has back reference to the now also archived parent region and if the snapshot is deleted they both gets cleaned up. Unfortunately the daughter regions only contains hfile links, so at this point the data is lost.

How to reproduce:

hbase shell <<EOF
create 'test', 'cf'
(0...2000).each{|i| put "test", "row#{i}", "cf:col", "val"}
flush 'test'
split 'test'
snapshot 'test', 'testshot'
EOF

This should make sure the snapshot is made before the compaction could be finished even with small amount of data.

sudo -u habse hbase org.apache.hadoop.hbase.snapshot.ExportSnapshot -snapshot testshot -copy-to hdfs://target:8020/apps/hbase/data/

I export the snapshot to make the usecase cleaner but deleting both the snapshot and the original table after the cloning should have the same effect.

clone_snapshot 'testshot', 'test2'
delete_snapshot "testshot"

I'm not sure what would be the best way to fix this. Preventing snapshots when a region is in split state, would make snapshot creation problematic. Forcing to run compaction as part of the split thread would make it rather slow. Propagating the daughter region references could prevent the deletion of the cloned parent region and the data would not be broken anymore but I'm not sure we have a logic in place that could pick up the pieces and finish the split process.