DOCS-3023: Corrected info in PR 3023 (#3231)

* DOCS-3023: Corrected info in PR 3023

Fixed migration troubleshooting section and removed existing troulbeshoot info from delta migration topic.

* Update migration-troubleshooting.md

@victor-v-rad, fixed for clarity, but am not sure I understand the second paragraph of the Explanation. Please clarify. thanks

* DOCS-3023: Corrected info in PR 3023

Improving description of how the delta mode works for migration

* DOCS-3023: Corrected info in PR 3023

Rewrote explanation for clarity.

* DOCS-3023: Corrected info in PR 3023

Fixed some GSP issues.

* DOCS-3023: Corrected info in PR 3023

Fixed some GSP issues

* DOCS-3023: Corrected info in PR 3023

Fixed links and code blocks

Author: bdenham

guides/v2.1/migration/migration-migrate-delta.md

@@ -11,7 +11,7 @@ functional_areas:
 
 ## Overview
 
-The incremental migration tool installs deltalog tables (with prefix m2_cl_*) and triggers to Magento 1 database during the [migration of data]({{ page.baseurl }}/migration/migration-migrate-data.html). It enables you to migrate only the changes made in Magento 1 since the last time you migrated data. These changes are:
+The incremental migration tool installs deltalog tables (with prefix `m2_cl_*`) and triggers (for tracking changes) in the Magento 1 database during the [migration of data]({{ page.baseurl }}/migration/migration-migrate-data.html). These deltalog tables and triggers are essential to ensuring that you migrate only the changes made in Magento 1 since the last time you migrated data. These changes are:
 
 * data that customers added via {% glossarytooltip 1a70d3ac-6bd9-475a-8937-5f80ca785c14 %}storefront{% endglossarytooltip %} (created orders, reviews, changes in customer profiles, etc.)
 
@@ -37,12 +37,6 @@ where
 
 * `{<path to config.xml>}` is the absolute file system path to `config.xml`; this argument is required.
 
-You must have the tables the migration initially created in the Magento 1 database in order to perform and retain the delta migration. If you wipe the Magento 1 database and try to do the delta, it will fail with the following error: 
-
-    Deltalog for customer_entity is not installed
-
-As an example, if you have Magento 1 on one server and Magento 2 on another that can't access the live Magento 1 database, you may need to copy the Magento 1 database over. In this case if you drop and overwrite the database, you will lose those special tables.
-
 {: .bs-callout .bs-callout-info }
 Incremental migration runs continuously until you stop it by pressing CTRL+C.
 

guides/v2.1/migration/migration-tool-internal-spec.md

@@ -284,7 +284,7 @@ Under node <code>&lt;value&gt;</code> there are rules that work with 'value' col
 
 ### Data migration mode
 
-In this mode most of the data will be migrated. Before data migration the integrity check stages run for each step. If integrity check passed the Data Migration Tool installs deltalog tables (with prefix m2_cl_*) and corresponding triggers to Magento 1 database. And runs data migration stage of steps. When migration is completed without errors the volume check checks data consistency. It can show a warning message if you migrate live store. Do not worry, delta migration will take care of this incremental data. Next the most valuable migration steps are described. It is Map Step, URL Rewrite Step, EAV Step.
+In this mode most of the data will be migrated. Before data migration the integrity check stages run for each step. If the integrity check passes, the Data Migration Tool installs deltalog tables (with prefix `m2_cl_*`) and corresponding triggers to the Magento 1 database and runs data migration stage of steps. When migration is completed without errors, the volume check checks data consistency. It can show a warning message if you migrate the live store. Do not worry, delta migration will take care of this incremental data. The most valuable migration steps are Map, URL Rewrite, and EAV.
 
 #### Map Step
 

guides/v2.2/migration/migration-tool-internal-spec.md

@@ -301,7 +301,7 @@ Under node <code>&lt;value&gt;</code> there are rules that work with 'value' col
 
 ### Data migration mode
 
-In this mode most of the data will be migrated. Before data migration the integrity check stages run for each step. If integrity check passed the Data Migration Tool installs deltalog tables (with prefix m2_cl_*) and corresponding triggers to Magento 1 database. And runs data migration stage of steps. When migration is completed without errors the volume check checks data consistency. It can show a warning message if you migrate live store. Do not worry, delta migration will take care of this incremental data. Next the most valuable migration steps are described. It is Map Step, URL Rewrite Step, EAV Step.
+In this mode most of the data will be migrated. Before data migration the integrity check stages run for each step. If the integrity check passes, the Data Migration Tool installs deltalog tables (with prefix `m2_cl_*`) and corresponding triggers to the Magento 1 database and runs data migration stage of steps. When migration is completed without errors, the volume check checks data consistency. It can show a warning message if you migrate the live store. Do not worry, delta migration will take care of this incremental data. The most valuable migration steps are Map, URL Rewrite, and EAV.
 
 #### Map Step
 

guides/v2.2/migration/migration-troubleshooting.md

@@ -1,10 +1,6 @@
 ---
 group: migration-guide
-subgroup: F_troubleshooting
 title: Troubleshooting
-menu_title: Troubleshooting
-menu_node: parent
-menu_order: 6
 ---
 
 ## Common error messages
@@ -13,13 +9,13 @@ This section is about the errors that might occur when you run the Data Migratio
 
 ### Source documents/fields not mapped
 
-{% highlight xml %}
+```xml
 Source documents are not mapped: <EXTENSION_TABLE>
-{% endhighlight %}
+```
 
-{% highlight xml %}
+```xml
 Source fields are not mapped. Document: <EXTENSION_TABLE>. Fields: <EXTENSION_FIELD>
-{% endhighlight %}
+```
 
 In rare cases, the message might mention `Destination documents` or `Destination fields` instead of source ones.
 
@@ -41,21 +37,21 @@ This message appears because the Data Migration Tool runs internal tests to veri
 
 To ignore database entities, add the `<ignore>` tag to an entity in the `map.xml` file, like this:
 
-{% highlight xml %}
+```xml
 <ignore>
    <field>sales_order_address_id</field>
 </ignore>
-{% endhighlight %}
+```
 
 {: .bs-callout .bs-callout-warning }
 Before ignoring entities by map file or using the `-a` option, make sure you do not need the affected data in your Magento 2 store.
 
 ### Class does not exist but mentioned
 
-{% highlight xml %}
+```xml
 Class <extension/class_name> does not exist but mentioned in:
 <eav_attribute.frontend_model> for <attribute_id=196>
-{% endhighlight %}
+```
 
 #### Explanation
 
@@ -75,11 +71,11 @@ A class from Magento 1 codebase could not be found in Magento 2 codebase during
 
 #### Error message text
 
-{% highlight xml %}
+```xml
 Foreign key <KEY_NAME> constraint fails.
 Orphan records id: <id_1>, <id_2> from <child_table>.
 <field_id> has no referenced records in <parent_table>
-{% endhighlight %}
+```
 
 #### Explanation
 
@@ -93,11 +89,11 @@ To keep the records, disable the `Data Integrity Step` by modifying the Data Mig
 
 ### Duplicates in URL rewrites
 
-{% highlight xml %}
+```xml
 There are duplicates in URL rewrites:
 Request path: towel.html Store ID: 2 Target path: catalog/product/view/id/10
 Request path: towel.html Store ID: 2 Target path: catalog/product/view/id/12
-{% endhighlight %}
+```
 
 #### Explanation
 
@@ -111,9 +107,9 @@ This configuration adds a hash-string to the conflicting records of {% glossaryt
 
 ### Mismatch of entities
 
-{%highlight xml%}
+```xml
 Mismatch of entities in the document: <DOCUMENT>
-{%endhighlight%}
+```
 
 #### Explanation
 
@@ -127,18 +123,16 @@ Run the Data Migration Tool in `Delta` mode to transfer incremental changes.
 
 ### Deltalog is not installed
 
-{%highlight xml%}
-Deltalog for <DATABASE> is not installed
-{%endhighlight%}
+```xml
+Deltalog for <TABLE_NAME> is not installed
+```
 
 #### Explanation
 
-This error occurs during the process of incremental migration of changes to data. It means the delta migration of changes made in Magento 1 since the last time you migrated data was not performed properly and retained.
+This error occurs during [incremental migration]({{ page.baseurl }}/migration/migration-migrate-delta.html) of changes to data. It means deltalog tables (with prefix `m2_cl_*`) were not found in Magento 1 database. The tool installs these tables during [data migration]({{ page.baseurl }}/migration/migration-migrate-data.html) as well as database triggers which track changes and fill deltalog tables.
 
-Delta migrations can fail when you wipe the Magento 1 database before doing the delta, since this action erases the tables created during the initial migration of data into the Magento 1 database. These tables are needed to perform the delta migration properly. 
+One reason for the error could be that you are trying to migrate from a *copy* of your live Magento 1 store, not from the live store itself. When you make a copy from a live Magento 1 store that has never been migrated, the copy does not contain the triggers and additional deltalog tables needed to complete a delta migration, so the migration fails. The Data Migration Tool does NOT make comparisons between the DB of M1 and M2 to migrate the differences. Instead, the tool uses the triggers and deltalog tables installed during the first migration in order to perform subsequent delta migrations. In such a case, your copy of the live Magento 1 DB will not contain the triggers and deltalog tables that the Data Migration Tool uses to perform a migration.
 
 #### Possible solution
 
-Do not wipe the Magento 1 database before the delta migration process is complete in order to keep your initial migration tables.
-
-If you have Magento 1 on one server and Magento 2 on another that can't access the live Magento 1 database, you may need to copy the Magento 1 database over first. Otherwise if you drop and overwrite the database, you will lose those special tables.
+We recommended testing the migration process from a copy of your Magento 1 database to fix your migration issues. After fixing the issues on the copy, start the migration process over again from your live Magento 1 database. This will help ensure a smooth migration process.