Introduction

This article will show you how to add a Fast Data Directory (FDD) to an existing forest.

Details

The fast data directory stores transaction journals and stands. When the directory becomes full, larger stands will be merged into the data directory. Once the size of the fast data directory approaches its limit, then stands are created in the data directory.

Although it is not possible to add an FDD path to a currently-existing forest, it is possible to do the following:

1. Destroy an existing forest configuration (while preserving the data)

2. Re-create a forest with the same name & data, with an FDD added

The queries below illustrate steps one and two of the process. Note that you can also do this with Admin UI.

The query below will delete the forest configurations but not data.

Preparation:

1. Schedule a downtime window for this procedure (DO NOT DO THIS ON A LIVE PRODUCTION SYSTEM)

2. Insure that all ingestion and merging has stopped

3. Just to be on safer side, take a Backup of the forest first before applying this in Production

3. Detach the forest before running these queries

1) Use the following API to Delete an existing forest configuration

NOTE: make sure to set the $delete-data papameter to false().

admin:forest-delete(
$config as element(configuration),
$forest-ids as xs:unsignedLong*,
$delete-data as xs:Boolean {=FALSE}
) as element(configuration)

2) Use the following API to create a new forest  pointing to the old data directory which includes the configured FDD:

admin:forest-create(
$config as element(configuration),
$forest-name as xs:string,
$host-id as xs:unsignedLong,
$data-directory as xs:string?,
[$large-data-directory as xs:string?],
[$fast-data-directory as xs:string?]
) as element(configuration)



Here's an example query that uses these APIs:

xquery version "1.0-ml";

declare namespace html = "http://www.w3.org/1999/xhtml";

import module namespace admin = "http://marklogic.com/xdmp/admin" 
at "/MarkLogic/admin.xqy";

let $config := admin:get-configuration()

(: preserve some path values from the old forest :)

let $forest-name := "YOUR_FOREST_NAME"

let $new-fast-data := "YOUR_NEW_FAST_DATA_DIR"

let $old-data := admin:forest-get-data-directory($config, admin:forest-get-id($config, $forest-name))

let $old-large-data := admin:forest-get-large-data-directory($config, admin:forest-get-id($config, $forest-name))

return
admin:save-configuration(admin:forest-delete(
$config, admin:forest-get-id($config, $forest-name),
fn:false())),

let $config1 := admin:get-configuration()
return
admin:save-configuration(admin:forest-create(
    $config1,
    $forest-name,
    xdmp:host(),
    $old-data,
    $old-large-data,
    $new-fast-data
))

You can create and attach the forest in a single transaction. This is also possible using the admin UI (as two separate transactions) i.e. deleting only configuration of forest without data.

After attaching the forest, please re-index and data will then migrate to FDD. Note that the sample query needs to be executed on the host where the forest resides.