[jira] Commented: (CASSANDRA-836) CommitLogSegment::seekAndWriteCommitLogHeader assumes header size doesn't change.

["Ross M (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-836?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12839485#action_12839485 ] 

Ross M commented on CASSANDRA-836:
----------------------------------

changing the size of the bitset is only one of the ways things could break. it's relying on the java BitSet::serialize to stay the same size. if http://java.sun.com/j2se/1.4.2/docs/api/serialized-form.html#java.util.BitSet is set in stone and all jdk impls have to implement it the same way it might be ok. if not things could break.

beyond that i was looking at trying to improve the on-disk size of things and this blocks me from being able to improve the size of BitSet's with a custom BitSetSerializer. currently a large percentage of the data is repetitive serialization info (java classes etc.) java serialization is not very space efficient. just take a good size commit log with varied data and gzip it, i was seeing ~10x size reduction. if you reduce the amount of data being written to the disk each write will take less time... 

if a header is going to be re-written like this it would be better to have the CommitLogHeader write all of it's data out on it's own so that it can make sure to use a fixed size rather than relying on the underlying implementations.


> CommitLogSegment::seekAndWriteCommitLogHeader assumes header size doesn't change.
> ---------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-836
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-836
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>         Environment: n/a - all
>            Reporter: Ross M
>            Priority: Minor
>         Attachments: BitSetSerializer.java
>
>
> CommitLogSegment::seekAndWriteCommitLogHeader assumes header size doesn't grow. there are pieces of the header (BitSet) that are serialized with java serialization which makes no such promises. 
> the following code:
>     /** writes header at the beginning of the file, then seeks back to current position */
>     void seekAndWriteCommitLogHeader(byte[] bytes) throws IOException
>     {
>         long currentPos = logWriter.getFilePointer();
>         logWriter.seek(0);
>         writeCommitLogHeader(bytes);
>         logWriter.seek(currentPos);
>     }
> works fine as long as the header size doesn't change, but if it grows the new header will over write the beginning of the data segment. the bit-set being written in the header happens to serialize to the same size, but there is no guarantee of this.
> i found this when looking at optimizing the serialization of data to disk (thus improving write throughput/performance.) i removed the ObjectOutputStream serialization in BitSetSerializer and replaced it with a custom serialization that omits the generic java serialization/ObjectOutputStream stuff and just writes on the "true" bits. the custom serialization worked fine, but broke other parts of the code when the header bitset had new bits turned on, thus growing the header's size, data segment bytes were overwritten.
> the serialized version of a BitSet can grow in a similar manner, no pomises of size/consistency are made, but with current use it luckily doesn't seem to happen.
> a good fix is unclear. without forcing the header to be a fixed/constant size in some manner this problem could pop up at any point. it's generally not safe to rewrite headers like this without custom code that ensures the size doesn't change. one fix would be to manually write all of the header data out (rather than relying on java serialization and serialization code in other parts of cassandra not to change.) another might be to pad the size of the header so that the data inside can grow, but that seems fraught with (potential) problems. (i've played around with padding the header length, but that seems to cause other things to break, which i haven't been able to track down yet.)

-- 
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.