Compressed Snapshot Files

When snapshot files are too large to fit on removable media or require transmission over slow networks, compressing the snapshot files is an option. Compressing snapshot files can reduce network traffic but it increases the time to generate and apply the snapshot.

Compression writes data in the Microsoft® CAB file format. You can compress snapshot files when you are saving them to an alternate location or when Subscribers are accessing them using FTP. Snapshot files written to the default snapshot folder on the Distributor cannot be compressed.

Disk Space Requirements

The amount of space required for a single snapshot can be affected by several factors including the size and number of articles published. You can create snapshot files in the default snapshot folder on the Distributor and in an alternate location. Compressing the snapshot files in the alternate location can reduce the overall space required.

When snapshot files are created in both the default directory and in an alternate location on the same drive, each file is created initially in the default directory and then copied to the alternate location. If you are using compressed snapshot files, the files are copied and compressed before they are placed in the alternate snapshot location. The total space required for all snapshot files in this situation is the size of the original snapshot files in the default location, plus the size of the compressed snapshot files in the alternate location.

If the alternate storage location is on a different drive than the default location, the space required at the default location is the size of the snapshot files. The space required at the alternate location is the total size of the compressed snapshot files.

When using only the alternate snapshot location, the Snapshot Agent writes files directly to that location. After the Snapshot Agent generates the files, the files are compressed by the CAB utility and become part of the compressed snapshot file with the extension .cab. After each file is compressed successfully and included in the compressed snapshot (.cab) file, the original, noncompressed file is deleted. The space required in the alternate location is the size of the last file in the default snapshot location (usually a .bcp file) plus the size of the compressed snapshot (.cab) file.

When the Subscriber receives a compressed snapshot file, the file is written initially to a temporary location. The default client working directory can be used, or an alternate location can be specified in the subscription properties. After the compressed snapshot file is copied to the Subscriber, the file is decompressed, in order, one file at a time by the CAB utility.

The uncompressed files are read by either the Merge Agent or the Distribution Agent and then executed or applied to the Subscriber. As each file is applied successfully, it is deleted and the next file in the snapshot directory is decompressed. Space required at the Subscriber is the size of the compressed snapshot file plus the largest uncompressed file.

To compress and deliver snapshot files