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Reduce Realm File Size - Kotlin SDK

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  • Automatic Compaction
  • Manual Compaction Options
  • Realm Configuration File
  • Realm.compactRealm Method
  • Make a Compacted Copy
  • Tips for Manually Compacting a Realm

Over time, the storage space used by Realm might become fragmented and take up more space than necessary. To rearrange the internal storage and potentially reduce the file size, the realm file needs to be compacted.

Realm's default behavior is to automatically compact a realm file to prevent it from growing too large. You can use manual compaction strategies when automatic compaction is not sufficient for your use case.

New in version 1.6.0.

The SDK automatically compacts Realm files in the background by continuously reallocating data within the file and removing unused file space. Automatic compaction is sufficient for minimizing the Realm file size for most applications.

Automatic compaction begins when the size of unused space in the file is more than twice the size of user data in the file. Automatic compaction only takes place when the file is not being accessed.

Manual compaction can be used for applications that require stricter management of file size or that use an older version of the SDK that does not support automatic compaction.

Realm reduces the file size by writing a new (compact) version of the file, and then replacing the original with the newly-written file. Therefore, to compact, you must have free storage space equivalent to the original realm file size.

You can configure realm to automatically compact the database each time a realm is opened, or you can compact the file without first obtaining a realm instance.

You can configure Realm to compact the realm file each time it is opened by setting a callback for the compactOnLaunch function for the configuration. When you call compactOnLaunch for the configuration, the DEFAULT_COMPACT_ON_LAUNCH_CALLBACK will trigger if the file is above 50 MB and 50% or more of the space in the realm file is unused. You can specify custom compaction settings when calling compactOnLaunch depending on your applications needs. The following code example shows how to do this:

// Set a max file size equal to 100MB in bytes
val maxFileSize = 100 * 1024 * 1024
val config = RealmConfiguration.Builder(setOf(King::class))
.compactOnLaunch{ totalBytes, usedBytes ->
// totalBytes refers to the size of the file on disk in bytes (data + free space)
// usedBytes refers to the number of bytes used by data in the file
// Compact if the file is over the max file size and less than 50% 'used'
(totalBytes > maxFileSize) && ((usedBytes / totalBytes) < 0.5)
val realm: Realm =

Alternatively, you can compact a realm file without having to open it by calling the compactRealm method:

val config = RealmConfiguration.create(schema = setOf(Item::class))
var compacted = Realm.compactRealm(config)

The compactRealm method will return true if the operation is successful and false if not.


compactRealm is not available on Windows (JVM), and will throw a NotImplementedError there.

You can save a compacted (and optionally encrypted) copy of a realm to another file location with the Realm.writeCopyTo method. The destination file cannot already exist.


Avoid calling writeCopyTo within a write transaction. If called within a write transaction, this method copies the absolute latest data. This includes any uncommitted changes you made in the transaction before this method call.

Manually compacting a realm can be a resource-intensive operation. Your application should not compact every time you open a realm. Instead, try to optimize compacting so your application does it just often enough to prevent the file size from growing too large. If your application runs in a resource-constrained environment, you may want to compact when you reach a certain file size or when the file size negatively impacts performance.

These recommendations can help you start optimizing compaction for your application:

  • Set the max file size to a multiple of your average realm state size. If your average realm state size is 10MB, you might set the max file size to 20MB or 40MB, depending on expected usage and device constraints.

  • As a starting point, compact realms when more than 50% of the realm file size is no longer in use. Divide the currently used bytes by the total file size to determine the percentage of space that is currently used. Then, check for that to be less than 50%. This means that greater than 50% of your realm file size is unused space, and it is a good time to compact. After experimentation, you may find a different percentage works best for your application.

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