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Storage space contentions on a database can result in the loss of critical data. To avoid this, administrators should keep a close watch on the space usage of each database, proactively detect a space crunch, and promptly resolve it. This is where the MongoDbSpaceTest test helps. This test auto-discovers all the databases on the MongoDB server and monitors the availability and space usage of each database. Alerts are promptly sent out if any database is found to be unavailable or is running out of space. In the event of a space crunch in a database, you can also use this test to figure out what is causing the space drain - documents? index files? or namespace files?

Outputs of the test : One set of results for each database in the target Mongo instance being monitored.

Measurement

Description

Measurement Unit

Interpretation

Is_db_available

Indicates whether/not this database is available.

The values that this measure can report and their corresponding numeric values are listed in the table below:

Measure Value	Numeric Value
Yes	1
No	0

Note:

By default, the measure reports the Measure Values listed in the table above to indicate the availability. However, in the graph of this measure, the same is indicated using the numeric equivalents only.

Data_file_size

Indicates the total size of this database. Note that this measure is applicable only if the monitored MongoDB server uses the MMAPv1 storage engine.

Storage_space

Indicates the total amount of space allocated to collections in this database for document storage.

Every MongoDB database instance consists of a namespace file, journal files and data files. Data files store BSON documents, indexes, and MongoDB-generated metadata in structures called extents. Each data file is made up of multiple extents. Extents are logical containers within data files used to store documents and indexes. Data and indexes are each contained in their own sets of extents. The value of this measure is equal to the size of all the data extents in a database.

Storage size does not decrease as you remove or shrink documents. This value may be smaller than the value of the Data_file_size measure for a database that is using the WiredTiger storage engine with compression enabled. For a database that is using the MMAPv1 storage engine on the other hand, this value may be larger than the value of the Data_file_size measure because it includes yet-unused space (in data extents) and space vacated by deleted or moved documents within extents.

Used_space

Indicates how much space allocated to this database is currently in use. Note that this measure is applicable only if the monitored MongoDB server uses the MMAPv1 storage engine.

Free_space

Indicates how much space allocated to this database is still available for use. Note that this measure is applicable only if the monitored MongoDB server uses the MMAPv1 storage engine.

Space_utilization

Indicates the percentage of allocated space that is used by this database. Note that this measure is applicable only if the monitored MongoDB server uses the MMAPv1 storage engine.

If the Space_utilization measure reports a value close to 100%, its a cause for concern, as it implies that your database is rapidly running out of space. Under such circumstances, you may want to consider allocating more space to this database.

Index_size

Indicates the total size of all indexes created on this database.

If the Space_utilization measure reports a value close to 100%, you may want to compare the Storage_space, Index_size, Namespace_file_size, or Data_file_size measures, to know where space is consumed the maximum.

Avg_Object_size

Indicates the average size of each document in this database.

If the Space_utilization of a database is increasing consistently along with the value of the Data_file_size measure, then, you may want to check the value of this measure. If the value of this measure is also very high, then you can conclude that the space usage is owing to the large size of documents in the database.

Namespace_file_size

Indicates the total size of all namespace files in this database.Note that this measure is applicable only if the monitored MongoDB server uses the MMAPv1 storage engine.

If the Space_utilization measure reports a value close to 100%, you may want to compare the Storage size, Index_size, Namespace file size, or Data_size measures, to know where space is consumed the maximum.

Number_of_extents

Indicates the total number of extents across all collections in this database.

Number

Extents are logical containers within data files used to store documents and indexes. Data and indexes are each contained in their own sets of extents.

Number_of_index

Indicates the total number of indexes across all collections in this database.

Number

Data_size

Indicates the total size of all the documents and padding stored in the database.

The value of this measure will decrease when you delete documents, but will not decrease when documents shrink because the space used by the original document has already been allocated (to that particular document) and cannot be used by other documents. Alternatively if a user updates a document with more data, the value of this measure will remain the same as long as the new document fits within its originally padded pre-allocated space.

Growth_rate

Indicates the rate at which the size of this database is growing.

The value of this measure is calculated using the following formula:

(Data size + Index size) of the current measurement period (-) (Data size + Index size) of the previous measurement period

A consistent increase in the value of this measure is a cause for concern, as it indicates a steady growth in database size.

Compression_ratio

Indicates the rate at which data in storage is compressed.

Percent

If the Growth_rate is very high and the Compression_ratio is very low, you may want to consider increasing the compression ratio to conserve storage space and optimize growth rate.