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Knowledgebase:
Reaching "stand limit" frequently?
30 June 2015 02:02 PM

Introduction

Seeing too many "stand limit" messages in your logs frequently? This article explains what this message means to your application and what actions should you take.

 

What are Stands and how their numbers can increase?

A stand holds a subset of the forest data and exists as a physical subdirectory under the forest directory. This directory contains a set of compressed binary files with names like TreeData, IndexData, Frequencies, Qualities, and such. This is where the actual compressed XMLdata (in TreeData) and indexes (in IndexData) can be found.

At any given time, a forest can have multiple stands. To keep the number of stands to a manageable level MarkLogic runs merges in the background. A merge takes some of the stands on disk and creates a new singular stand out of them, coalescing and optimizing the indexes and data, as well as removing any previously deleted fragments.

MarkLogic Server has a fixed limit for the maximum number of stands (64). When that limit is reached you will no longer be able to update your system. While MarkLogic automatically manage merges and it is unlikely to reach this limit, there are few configurations under user control that may impact merges and you may see this issue. e.g.

1.) You can manage merges using Merge Policy Controls. e.g. setting a low merge max size would stop merges beyond the configured size and hence the overall number of stands would keep growing.

2.) Low value of background-io-limit would mean less amount of I/O for background tasks such as merges. This may also adversely affect the merge rate and hence the number of stands may grow.

3.) Low in-memory settings not keeping up with an aggressive data load. e.g. If you are bulk loading large documents and have low in memory tree size then stands may accumulate and reach the hard limit.

 

What you can do to keep the number of stands within manageable limit?

While MarkLogic automatically manage merges to keep the number of stands at a manageable level, it adds WARNING entry to the logs when it sees the number of stands growing alarmingly! e.g. Warning: Forest XXXXX is at 92% of stand limit

If you see such messages in your logs, you should take some action as reaching the hard limit of 64 would mean you will no longer be able to update your system.

Here's what you can check and do to lower the number of stands.

1.) If you have configured merge policy controls then check if they actually match with your application usage. You could change the required settings as needed. For instance:

2.) There should be no merge blackouts during ingestion, or any time there is heavy updating of your content.

3.) Beginning with MarkLogic version 7, the server is able to manage merges with less free space required on your drives (1.5 times the size of your content). This is accomplished by setting the merge max size to 32768 (32GB). Although this does create more stands, this is OK on newer systems, since the server is able to use extra CPU cores in parallel.

2.) If you have configured background-io-limit then check if that is sufficient for your application usage. If needed, increase the value so that merges can make use of more IO. You should only use this setting on systems that have limited disk IO. In general you want to first set it to 200, and if the disk IO seems to still be overwhelmed, set it to 150 and so on. A setting of 1oo may be too low for systems that are doing ingestion, since the merge process needs to be able to keep up with stand creation.

3.) If you are performing bulk loads then check if the in-memory settings are suffificient and can be increased. If needed, increase the required value so that in-memory stands (and as a result on-disk stands) accomodate more data and thereby decreases the number of stands. If you do grow the in-memory caches, make sure to grow the database journal files by a corresponding amount. This will insure that a single large transaction will be able to fit in the journals.

 

Conclusion 

If you decide to control MarkLogic's merge process, you should monitor the system for any adverse effect that it may cause and take actions accordingly. MarkLogic Server continuously assesses the state of each database and the default merge settings and the dynamic nature of merges will keep the database tuned optimally at all times. So if you are unsure - let MarkLogic handle the merges for you!

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