[
https://issues.apache.org/jira/browse/HDFS-3510?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Colin Patrick McCabe updated HDFS-3510:
---------------------------------------
Attachment: HDFS-3510-b1.002.patch
* fill with all 0xff, so that we don't have to handle OP_INVALID specially.
Also, technically it is undefined what byte pattern ByteBuffer.allocateDirect
fills the buffer with, although it is 0 in practice in Oracle implementations.
In JDK7 they specified that it zero-fills.
> FSEditLog pre-allocation does not work in branch-1
> --------------------------------------------------
>
> Key: HDFS-3510
> URL: https://issues.apache.org/jira/browse/HDFS-3510
> Project: Hadoop HDFS
> Issue Type: Bug
> Affects Versions: 1.0.0
> Reporter: Colin Patrick McCabe
> Assignee: Colin Patrick McCabe
> Fix For: 1.0.0
>
> Attachments: HDFS-3510-b1.001.patch, HDFS-3510-b1.002.patch
>
>
> In the FSEditLog, we want to avoid running out of space in the middle of
> writing an edit log operation to the disk. We do this by a process called
> "preallocation"-- reserving space on the disk for the upcoming edit log
> entries before beginning to write them.
> branch-1 has some major problems with the way it does preallocation. These
> problems can lead to corrupt edit logs when the disk runs out of space during
> an edit log sync operation.
> The problems are:
> * We use FileChannel#write without checking for short writes, but
> WritableByteChannel explicitly documents that they are possible, and the
> FileChannel subclass is silent on the issue.
> * We only try to do preallocation when the current position is less than 4096
> bytes from the end of the file. However, bufReady starts out at 512kb, and
> only gets bigger from there. There is no way that 4kb is enough space to
> reserve.
> * The current code seems to be based on a misunderstanding of how space is
> allocated in files in Linux. In FileChannel#write(ByteBuffer, long), the
> second argument is the offset to start writing at. Since we set this to
> fc.position() + 1024*1024, this means that we *start* writing a megabyte
> after the end of the file. This is guaranteed to create a sparse file on
> Linux, defeating the point of pre-allocation.
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators:
https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira
