Enis Soztutar commented on HBASE-16820:

bq. I think It is not wrong to use the flushseqid as the edits in bulk loaded 
file's seqid. 
See the reasoning in the description. We should not use the same sequence id 
for two different purposes. 

bq. if I remember from the original implementation, we used the flush 
sequenceid because that was guaranteed 'safe'; going beyond this id there were 
concerns some other edit could sneak in between the flush and bulk load. 
Checked the code. It seems that we are not acquiring the updatesLock for BL 
which means that we can end up with the case that edits can come in between the 
flush seq id and BL sequence id. We can make it so that we get the updatesLock 
first before the flush, then after flush, we get the BL sequence id. Since the 
updates lock is reentrant, it should work as long as the same thread is doing 
the flush.  

> BulkLoad mvcc visibility only works accidentally 
> -------------------------------------------------
>                 Key: HBASE-16820
>                 URL: https://issues.apache.org/jira/browse/HBASE-16820
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Enis Soztutar
>            Assignee: Enis Soztutar
>         Attachments: HBASE-16820-branch-1.1-v0.patch
> [~sergey.soldatov] has been debugging an issue with a 1.1 code base where the 
> commit for HBASE-16721 broke the bulk load visibility. After bulk load, the 
> bulk load files is not visible because the sequence id assigned to the bulk 
> load is not advanced in mvcc. 
> Debugging further, we have noticed that bulk load behavior is wrong, but it 
> works "accidentally" in all code bases (but broken in 1.1 after HBASE-16721). 
> Let me explain: 
>  - BL request can optionally request a flush before hand (this should be the 
> default) which causes the flush to happen with some sequenceId. The flush 
> sequence id is one past all the cells' sequenceids. This flush sequence id is 
> returned as a result to the flush operation. 
>  - BL then uses this particular sequenceId to mark the files, but itself does 
> not get a new sequenceid of its own, or advance the mvcc number. 
>  - BL completes WITHOUT making sure that the sequence id is visible. 
>  - BL itself though writes entries to the WAL for the BL event, which in 1.2 
> code bases goes through the whole mvcc + seqId paths, which makes sure that 
> earlier sequenceIds (the flush sequenceId) are visible via mvcc. 
> The problem with 1.1 is that the WAL entries only get sequence ids, but do 
> not touch mvcc. With the patch for HBASE-16721, we have made it so that the 
> flushedSequenceId is not used in mvcc as the highest read point (although all 
> the data is still visible).
> BL relying on the flush sequence id is wrong for two reasons: 
>  - BL files are loaded with the flush sequence id from the memstore. This 
> particular sequence id is used twice for two different things and ends up 
> being the sequence id for flushed file as well as BL'ed files. 
>  - BL should make sure that it gets a new sequence id and that sequence id is 
> visible before returning the results. 
> [~ndimiduk] FYI. 

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