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https://issues.apache.org/jira/browse/HBASE-15236?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15236139#comment-15236139
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stack commented on HBASE-15236:
-------------------------------
What a dirty bug. That is some pretty amazing digging on your part [~appy]
figuring out the root cause. You the man.
On the patch, from your finding, seems like the KeyValueScanner#getSequenceId
was a bit wrong-headed in its original implementation... or at least, it can
get messy as you've found trying to stamp a KVScanner with a 'sequenceId'. (A
KVScanner 'readPoint' seems more natural but no reason two layers of KVScanners
couldn't have same readPoint so that won't help you here).
You use the notion of priority... of one KVScanner having priority over another
KVScanner. While 'correct', when fellas see priority in hbase codebase, they
are thinking scheduling. Suggest you use another term... I was thinking
'ordinal' as in the position the KVScanner holds in the tiering of
KVScanners... but is it true that two KVScanners might return same ordinal or
that ordinal is not always available when scanning? (memstore would be at
position 0, so its ordinal would be zero, and so on back through the HFiles...
though in the patch memstore is MAX_INT so I have the wrong order here....).
Why is the MemStore#getPriorityID synchornized? Does the value change post
construction of the MemStore object?
For Segments, can we not set the 'priority' on construction rather than open up
a setPriorityID? As you do for StoreFileScanner.
Suggest you also say why we have this priority id at all in the KVScanner#get*
method.... its to break ties.
If every Cell had a sequenceId, we'd not need this?
> Inconsistent cell reads over multiple bulk-loaded HFiles
> --------------------------------------------------------
>
> Key: HBASE-15236
> URL: https://issues.apache.org/jira/browse/HBASE-15236
> Project: HBase
> Issue Type: Bug
> Reporter: Appy
> Assignee: Appy
> Attachments: HBASE-15236-master-v2.patch,
> HBASE-15236-master-v3.patch, HBASE-15236-master-v4.patch,
> HBASE-15236-master-v5.patch, HBASE-15236-master-v6.patch,
> HBASE-15236-master-v7.patch, HBASE-15236.patch, TestWithSingleHRegion.java,
> tables_data.zip
>
>
> If there are two bulkloaded hfiles in a region with same seqID, same
> timestamps and duplicate keys*, get and scan may return different values for
> a key. Not sure how this would happen, but one of our customer uploaded a
> dataset with 2 files in a single region and both having same bulk load
> timestamp. These files are small ~50M (I couldn't find any setting for max
> file size that could lead to 2 files). The range of keys in two hfiles are
> overlapping to some extent, but not fully (so the two files are because of
> region merge).
> In such a case, depending on file sizes (used in
> StoreFile.Comparators.SEQ_ID), we may get different values for the same cell
> (say "r", "cf:50") depending on what we call: get "r" "cf:50" or get "r"
> "cf:".
> To replicate this, i ran ImportTsv twice with 2 different csv files but same
> timestamp (-Dimporttsv.timestamp=1). Collected two resulting hfiles in a
> single directory and loaded with LoadIncrementalHFiles. Here are the commands.
> {noformat}
> //CSV files should be in hdfs:///user/hbase/tmp
> sudo -u hbase hbase org.apache.hadoop.hbase.mapreduce.ImportTsv
> -Dimporttsv.columns=HBASE_ROW_KEY,c:1,c:2,c:3,c:4,c:5,c:6
> -Dimporttsv.separator='|' -Dimporttsv.timestamp=1
> -Dimporttsv.bulk.output=hdfs:///user/hbase/1 t tmp
> {noformat}
> tables_data.zip contains three tables: t, t2, and t3.
> To load these tables and test with them, use the attached
> TestWithSingleHRegion.java. (Change ROOT_DIR and TABLE_NAME variables
> appropriately)
> Hfiles for table 't':
> 1) 07922ac90208445883b2ddc89c424c89 : length = 1094: KVs = (c:5, 55) (c:6, 66)
> 2) 143137fa4fa84625b4e8d1d84a494f08 : length = 1192: KVs = (c:1, 1) (c:2, 2)
> (c:3, 3) (c:4, 4) (c:5, 5) (c:6, 6)
> Get returns 5 where as Scan returns 55.
> On the other hand if I make the first hfile, the one with values 55 and 66
> bigger in size than second hfile, then:
> Get returns 55 where as Scan returns 55.
> This can be seen in table 't2'.
> Weird things:
> 1. Get consistently returns values from larger hfile.
> 2. Scan consistently returns 55.
> Reason:
> Explanation 1: We add StoreFileScanners to heap by iterating over list of
> store files which is kept sorted in DefaultStoreFileManger using
> StoreFile.Comparators.SEQ_ID. It sorts the file first by increasing seq id,
> then by decreasing filesize. So our larger files sizes are always in the
> start.
> Explanation 2: In KeyValueHeap.next(), if both current and this.heap.peek()
> scanners (type is StoreFileScanner in this case) point to KVs which compare
> as equal, we put back the current scanner into heap, and call pollRealKV() to
> get new one. While inserting, KVScannerComparator is used which compares the
> two (one already in heap and the one being inserted) as equal and inserts it
> after the existing one. \[1\]
> Say s1 is scanner over first hfile and s2 over second.
> 1. We scan with s2 to get values 1, 2, 3, 4
> 2. see that both scanners have c:5 as next key, put current scanner s2 back
> in heap, take s1 out
> 4. return 55, advance s1 to key c:6
> 5. compare c:6 to c:5, put back s2 in heap since it has larger key, take out
> s1
> 6. ignore it's value (there is code for that too), advance s2 to c:6
> Go back to step 2 with both scanners having c:6 as next key
> This is wrong because if we add a key between c:4 and c:5 to first hfile, say
> (c:44, foo) which makes s1 as the 'current' scanner when we hit step 2, then
> we'll get the values - 1, 2, 3, 4, foo, 5, 6.
> (try table t3)
> Fix:
> Assign priority ids to StoreFileScanners when inserting them into heap,
> latest hfiles get higher priority, and use them in KVComparator instead of
> just seq id.
> \[1\]
> [PriorityQueue.siftUp()|http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/PriorityQueue.java#586]
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