But again, it causes problems when things go wrong. It is what is called when the above mentioned modification methods are invoked We are talking about fsync style issues.
What we are missing though is where the KeyValue belongs to, i. If you do this for every region separately this would not scale well - or at least be an itch that sooner or later is causing pain.
With Hadoop 0.But that was not the case. What is left is to improve how the logs are split to make the process faster. By default you certainly want the WAL, no doubt about that. That in reality this is all a bit more complicated is discussed below. Here is how is the BigTable addresses the issue: One approach would be for each new tablet server to read this full commit log file and apply just the entries needed for the tablets it needs to recover. What is also stored is the above sequence number. Columns contain cells, which are timestamped versions of the value in that column. Only those with edits need to wait then until the logs are split.
The image to the right shows three different regions. To learn more about Azure-managed disks, see Introduction to Azure managed disks. However, under such a scheme, if machines were each assigned a single tablet from a failed tablet server, then the log file would be read times once by each server.So if the server crashes it can effectively replay that log to get everything up to where the server should have been just before the crash. Also we want to make sure a log is persisted on a regular basis. So either the logs are considered full or when a certain amount of time has passed causes the logs to be switched out, whatever comes first. By default this is set to 1 hour. The choice is yours. HBASE made the class implementing the log configurable. Planned Improvements For HBase 0. The old logs usually come from a previous region server crash. As far as HBase and the log is concerned you can turn down the log flush times to as low as you want - you are still dependent on the underlaying file system as mentioned above; the stream used to store the data is flushed but is it written to disk yet?
To mitigate the issue the underlaying stream needs to be flushed on a regular basis. What you may have read in my previous post and is also illustrated above is that there is only one instance of the HLog class, which is one per HRegionServer. But if you have to split the log because of a server crash then you need to divide into suitable pieces, as described above in the "replay" paragraph.
With each record that number is incremented to be able to keep a sequential order of edits. This functionality is provided by the LogFlusher class and thread.
HBASE made the class implementing the log configurable.