HBase is interesting here, because it rewrites old data into new files. So a power outage by default would not just lose new data but potentially old data as well.
You can enable "sync on block close" in HDFS, and then at least be sure that closed blocks (and thus files) are synced to disk physically.
I found that if that is paired with the "sync behind write" fadvice hint there performance impact is minimal.
Dave Latham <[EMAIL PROTECTED]> wrote:
>Thanks for the response, Suresh.
>I'm not sure that I understand the details properly. From my reading of
>HDFS-744 the hsync API would allow a client to make sure that at any point
>in time it's writes so far hit the disk. For example, for HBase it could
>apply a fsync after adding some edits to its WAL to ensure those edits are
>fully durable for a file which is still open.
>However, in this case the dfs file was closed and even renamed. Is it the
>case that even after a dfs file is closed and renamed that the data blocks
>would still not be synced and would still be stored by the datanode in
>"blocksBeingWritten" rather than in "current"? If that is case, would it
>be better for the NameNode not to reject replicas that are in
>blocksBeingWritten, especially if it doesn't have any other replicas
>On Mon, Jul 1, 2013 at 3:16 PM, Suresh Srinivas <[EMAIL PROTECTED]>wrote:
>> Yes this is a known issue.
>> The HDFS part of this was addressed in
>> https://issues.apache.org/jira/browse/HDFS-744 for 2.0.2-alpha and is not
>> available in 1.x release. I think HBase does not use this API yet.
>> On Mon, Jul 1, 2013 at 3:00 PM, Dave Latham <[EMAIL PROTECTED]> wrote:
>> > We're running HBase over HDFS 1.0.2 on about 1000 nodes. On Saturday the
>> > data center we were in had a total power failure and the cluster went
>> > hard. When we brought it back up, HDFS reported 4 files as CORRUPT. We
>> > recovered the data in question from our secondary datacenter, but I'm
>> > trying to understand what happened and whether this is a bug in HDFS that
>> > should be fixed.
>> > From what I can tell the file was created and closed by the dfs client
>> > (hbase). Then HBase renamed it into a new directory and deleted some
>> > files containing the same data. Then the cluster lost power. After the
>> > cluster was restarted, the datanodes reported into the namenode but the
>> > blocks for this file appeared as "blocks being written" - the namenode
>> > rejected them and the datanodes deleted the blocks. At this point there
>> > were no replicas for the blocks and the files were marked CORRUPT. The
>> > underlying file systems are ext3. Some questions that I would love get
>> > answers for if anyone with deeper understanding of HDFS can chime in:
>> > - Is this a known scenario where data loss is expected? (I found
>> > HDFS-1539 but that seems different)
>> > - When are blocks moved from blocksBeingWritten to current? Does that
>> > happen before a file close operation is acknowledged to a hdfs client?
>> > - Could it be that the DataNodes actually moved the blocks to current
>> > after the restart ext3 rewound state somehow (forgive my ignorance of
>> > underlying file system behavior)?
>> > - Is there any other explanation for how this can happen?
>> > Here is a sequence of selected relevant log lines from the RS (HBase
>> > Region Server) NN (NameNode) and DN (DataNode - 1 example of 3 in
>> > question). It includes everything that mentions the block in question in
>> > the NameNode and one DataNode log. Please let me know if this more
>> > information that would be helpful.
>> > RS 2013-06-29 11:16:06,812 DEBUG org.apache.hadoop.hbase.util.FSUtils:
>> > Creating
>> > with permission=rwxrwxrwx
>> > NN 2013-06-29 11:16:06,830 INFO org.apache.hadoop.hdfs.StateChange: