[jira] [Comment Edited] (ACCUMULO-578) consider using hdfs for the walog

["Eric Newton (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ACCUMULO-578?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13280091#comment-13280091 ] 

Eric Newton edited comment on ACCUMULO-578 at 5/21/12 1:58 PM:
---------------------------------------------------------------

The loggers implemented another feature I forgot about which was to atomically determine which logs were not open, and therefore available for collection.  We need a new way to determine this.

# open the file and begin using it, in a directory named for the tserver id
# write references to the log into the !METADATA table, as the log is used
# tablet server removes references to logs as tablets flush to disk
# tserver deletes the log when the last reference is removed
# gc doesn't collect files with good tserver ids or METADATA table references
# master will assign log sorts when it finds an unassigned tablet with log references
# once a tablet's logs have been sorted, the tablet is assigned by the master
# gc will remove sorted log entries when all references to the logs have been removed

For backwards compatibility, tservers will copy/sort any existing logger-based WALs to the recovery area when they start.

                
      was (Author: ecn):
    The loggers implemented another feature I forgot about which was to atomically determine which logs were not open, and therefore available for collection.  We need a new way to determine this.

h3. One approach:
# write references to the log into the !METADATA table, along with a tserver id
# open the file and begin using it
# when the tablet server closes the log, it removes the tserver id
# gc doesn't collect files with good tserver ids or METADATA table references

h3. Bonus points:
# move the majority of log file collection to the tablet servers: they know when their tablets have lost references to the log
# master/tservers can sort the logs during recovery, and delete the unsorted copy
# tablet servers can gc the recovery log, after a check of the METADATA table
#* Need to avoid problems like ACCUMULO-598
                  
> consider using hdfs for the walog
> ---------------------------------
>
>                 Key: ACCUMULO-578
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-578
>             Project: Accumulo
>          Issue Type: Improvement
>          Components: logger, tserver
>    Affects Versions: 1.5.0-SNAPSHOT
>            Reporter: Eric Newton
>            Assignee: Eric Newton
>         Attachments: comparison.png
>
>
> Using HDFS for walogs would fix:
>  * ACCUMULO-84: any node can read the replicated files
>  * ACCUMULO-558: wouldn't need to monitor loggers
>  * ACCUMULO-544: log references wouldn't include hostnames
>  * ACCUMULO-423: wouldn't need to monitor loggers
>  * ACCUMULO-258: hdfs has load balancing already
> To implement it, we would need the ability to distribute log sorts.
> Continuing to use loggers helps us avoid:
>  * hdfs pipeline strategy
>  * we don't have fine-grained insight when a single node makes dfs slow
>  * additional namenode pressure
>  * flexibility: for example, we can add fadvise() calls to the logger before HDFS supports it

--
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