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Author: sreekzz

articles/hdinsight/hbase/Troubleshoot-data-retention-(TTL)-issues-with-expired-data-not-being-deleted-from-storage.md

@@ -1,36 +1,38 @@
 ---
-title: Troubleshoot Apache HBase performance issues on Azure HDInsight
-description: Troubleshoot various Apache HBase performance tuning guidelines and tips for getting optimal performance on Azure HDInsight. 
+title: Troubleshoot data retention (TTL) issues with expired data not being deleted from storage on Azure HDInsight
+description: Troubleshoot various data-retention (TTL) issues with expired data not being deleted from storage on Azure HDInsight
 ms.service: hdinsight
 ms.topic: troubleshooting
 ms.date: 05/06/2022
 ---
 
-In HBase cluster, you may decide that you would like to remove data after it ages either to free some storage and save on costs as the older data is no longer needed, either to comply with regulations. When that is needed , you will usually set TTL in a table at the ColumnFamily level to expire and automatically delete older data. While TTL can be set as well at cell level, setting it at ColumnFamily level is usually a more convenient option because the ease of administration and because a cell TTLs (expressed in ms)cannot extend the effective lifetime of a cell beyond a ColumnFamily level TTL setting (expressed in seconds), so only required shorter retention times at cell level could benefit from setting cell level TTL. 
+# Troubleshoot data retention (TTL) issues with expired data not being deleted from storage on Azure HDInsight
+
+In HBase cluster, you may decide that you would like to remove data after it ages either to free some storage and save on costs as the older data is no longer needed, either to comply with regulations. When that is needed, you'll usually set TTL in a table at the ColumnFamily level to expire and automatically delete older data. While TTL can be set as well at cell level, setting it at ColumnFamily level is usually a more convenient option because the ease of administration and because a cell TTLs (expressed in ms) can't extend the effective lifetime of a cell beyond a ColumnFamily level TTL setting (expressed in seconds), so only required shorter retention times at cell level could benefit from setting cell level TTL. 
 
 Despite setting TTL, you may notice sometimes that you don't obtain the desired effect, i.e. some data hasn't expired and/or storage size hasn't decreased.
 
 ## Prerequisites:
 
 To prepare to follow the steps and commands below, open 2 ssh connections to HBase cluster:
-1)  In one of the ssh sessions keep the default bash shell;
+1)  In one of the ssh sessions keeps the default bash shell;
 2)  In the second ssh session launch HBase shell by running the command below:
 
 ```
 hbase shell
 ```
 ### Check if desired TTL is configured and if expired data is removed from query result
 
-Follow the steps below to understand where is the issue. Start by checking if he the behavior occurs for a specific table or for all the tables. If you are unsure whether the issue impacts all the tables or a specific table, just consider as example a specific table name for the start.  
-1) Check first that TTL has been configured for ColumnFamily for the target tables. Run the command below in the ssh session where you launched HBase shell and observe example and output below. One column family has TTL set to 50 seconds, the other ColumnFamily has no value configured for TTL, thus it appears as "FOREVER" (data in this column family is not configured to expire):
+Follow the steps below to understand where is the issue. Start by checking if he the behavior occurs for a specific table or for all the tables. If you're unsure whether the issue impacts all the tables or a specific table, just consider as example a specific table name for the start.  
+1) Check first that TTL has been configured for ColumnFamily for the target tables. Run the command below in the ssh session where you launched HBase shell and observe example and output below. One column family has TTL set to 50 seconds, the other ColumnFamily has no value configured for TTL, thus it appears as "FOREVER" (data in this column family isn't configured to expire):
 
   ```
   describe 'table_name'
   ```
 
 2)  If not configured, default TTL is set to 'FOREVER'. There are 2 possibilities why data is not expired as expected and removed from query result:
-* a)  If TTL has any other value than 'FOREVER', observe the value for column family and note down the value in seconds(pay special attention to value correlated with the unit measure as cell TTL is in ms, but column family TTL is in seconds) to confirm if it is the expected one. If the observed value is not correct, fix that first.
-* b) If TTL value is 'FOREVER' for all column families, configure TTL as first step and afterwards monitor if data is expired as expected.
+a)  If TTL has any other value then 'FOREVER', observe the value for column family and note down the value in seconds(pay special attention to value correlated with the unit measure as cell TTL is in ms, but column family TTL is in seconds) to confirm if it is the expected one. If the observed value isn't correct, fix that first.
+b) If TTL value is 'FOREVER' for all column families, configure TTL as first step and afterwards monitor if data is expired as expected.
 3) If you establish that TTL is configured and has the correct value for the ColumnFamily, next step is to confirm that the expired data no longer shows up when doing table scans. When data expires, it should be removed and not show up in the scan table results. Run the below command in HBase shell to check:
 
   ```	
@@ -52,7 +54,7 @@ Follow the steps below to understand where is the issue. Start by checking if he
 
 ### Check the number and size of StoreFiles per table per region after flush
 
-6)  Based on the TTL configured for each ColumnFamily and how much data is written in the table for the target ColumnFamily, part of the data may still exist in MemStore and is not written as StoreFile to storage. Thus, to make sure that the data is written to storage as StoreFile, before the maximum configured MemStore size is reached, you can run the following command in HBase shell to write data from MemStore to StoreFile immediately. 
+6)  Based on the TTL configured for each ColumnFamily and how much data is written in the table for the target ColumnFamily, part of the data may still exist in MemStore and isn't written as StoreFile to storage. Thus, to make sure that the data is written to storage as StoreFile, before the maximum configured MemStore size is reached, you can run the following command in HBase shell to write data from MemStore to StoreFile immediately. 
 
   ```
 flush 'table_name'
@@ -64,9 +66,9 @@ flush 'table_name'
 ### Check the number and size of StoreFiles per table per region after major compaction
 
 
-8) 	At this point, the data from MemStore has been written to StoreFile, in storage, but expired data may still exist in one or more of the current StoreFiles. Although minor compactions can help delete some of the expired entries, it is not guaranteed that it will remove all of them as minor compaction will usually not select all the StoreFiles for compaction, while major compaction will select all the StoreFiles for compaction in that region. 
+8) 	At this point, the data from MemStore has been written to StoreFile, in storage, but expired data may still exist in one or more of the current StoreFiles. Although minor compactions can help delete some of the expired entries, it is'nt guaranteed that it will remove all of them as minor compaction will usually not select all the StoreFiles for compaction, while major compaction will select all the StoreFiles for compaction in that region. 
 
-       Also, there is another situation when minor compaction may not remove cells with TTL expired. There is a property named MIN_VERSIONS and it defaults to 0 only (see in the above output from describe 'table_name' the property MIN_VERSIONS=>'0'). If this property is set to 0, the minor compaction will remove the cells with TTL expired. If this value is greater than 0, minor compaction may not remove the cells with TTL expired even if it touches the corresponding file as part of compaction. This property configures the min number of versions of a cell to keep, even if those versions have TTL expired.
+       Also, there's another situation when minor compaction may not remove cells with TTL expired. There's a property named MIN_VERSIONS and it defaults to 0 only (see in the above output from describe 'table_name' the property MIN_VERSIONS=>'0'). If this property is set to 0, the minor compaction will remove the cells with TTL expired. If this value is greater than 0, minor compaction may not remove the cells with TTL expired even if it touches the corresponding file as part of compaction. This property configures the min number of versions of a cell to keep, even if those versions have TTL expired.
 
 9) To make sure expired data is also deleted from storage, we need to run a major compaction operation. The major compaction operation, when completed, will leave behind a single StoreFile per region. In HBase shell, run the command to execute a major compaction operation on the table:
 
@@ -75,7 +77,7 @@ flush 'table_name'
 	major_compact 'table_name'
   ```
 
-10) Depending on the table size, major compaction operation can take some time. Use the command below in HBase shell to monitor progress. If the compaction is still running when you execute the command below, you will see the output "MAJOR", but if the compaction is completed, you will see the output "NONE":
+10) Depending on the table size, major compaction operation can take some time. Use the command below in HBase shell to monitor progress. If the compaction is still running when you execute the command below, you'll see the output "MAJOR", but if the compaction is completed, you will see the output "NONE":
 
   ```
 	compaction_state 'table_name'