Data Deduplication
Imagine that you are building a small application that allows users to upload small text files containing greetings (or other types of files). Chances are good that multiple users will upload the exact same greeting messages. Thus, your storage will eventually contain duplicates.
To avoid this kind of situation, the database adapter makes use of data deduplication: "[...] a technique for eliminating duplicate copies of repeating data [...]" (Wiki) Consider the following:
$content = 'Hi there';
$filesystem->write('home/message.txt', $content);
$filesystem->write('home/other_message.txt', $content);
In the above example, two files are written to the database. However, the adapter ensures that the content is only stored once.
Content Hashing
Data deduplication is achieved by hashing a file's content and checking if that hash already exists. If that is the case, then an internal reference_count is incremented. Content is not inserted if this is the case. However, if the hash does not exist, then content is inserted into the database.
By default, sha256 hashing algorithm is used when hashing contents. You can change this via the setHashAlgorithm():
$adapter->setHashAlgorithm('md5')
Caution
Be careful of what hashing algorithm you choose. Some are fast, but might have a high risk of hash-collision. See wiki for more information.
When files are deleted
The adapter automatically cleans up its file-content records, when a file is requested deleted. In this process, the internal reference_count is decreased when a file is deleted. When the reference counter reaches 0, actual content is automatically removed.
Performance Considerations
Due to this applied deduplication technique, this adapter SHOULD NOT be expected to be very fast. It will get the job done. But, if performance is very important for your application, then you SHOULD choose a different flysystem adapter!