Managing Cache Files

When downloading image information for linked images created from OGC WMS image servers, Manifold Image Servers or from TerraServer servers, Manifold provides a cache data option (on by default) to cache image tiles that are fetched on local hard disk. To prevent cache files from growing without limit, users must clean out old cache files from time to time.


The Cache data option is important because it keeps a copy of the linked image in local disk cache. If this option is on, the next time we open a project containing a linked image, Manifold will be able to use the copy of the image saved in the local disk cache and will not have to go through the process of downloading the image again. The data cache will persist across sessions and projects so that once a tile is downloaded it will not have to be downloaded again. If we have a large hard disk with many gigabytes of storage available we can cache large numbers of large images.


The location of the cache folder is at what ever location is specified in the Data Cache item in the Tools - Options - File Locations dialog, with WMS cache files being stored in a series of subfolders and TerraServer cache files being stored in a TerraServer subfolder. The default value for the data cache location is %MyDocuments%, so by default each user's files will be stored in the user's own cache folder.


When multiple users are working it might make sense to modify the default Data Cache setting for each user so the cache folder location is the same for all users and is on a shared resource available from all machines on the local network. For example, if there is a machine called Storage on our local network with a Cache folder on its C: drive, we might use a Data Cache setting of \\Storage\C\Cache for all users. In that case, any tile files that are retrieved by one user will be available to all users so that tile files for a given view will never have to be fetched twice over the network. This assumes, of course, that the network is fast enough so that use of cache over the network will not be too slow.


It is also important when working with large linked images that the location specified for the cache is on a disk drive with plenty of free space. Users sometimes will ask for download of very large linked images without thinking about how large the resultant image may be and thus end up inadvertently requesting immensely large data sets.


The data cache folder may be moved to a different location so long as the path specified in the Data Cache option is adjusted to point to the new location.


Cleaning Out Old Cache Files


Cache files persist forever until they are manually deleted. Manifold will not clean out cache files or automatically delete older cache files because cache files may be in use by multiple users if they are in shared storage. To avoid running out of free disk space, users should manually delete unnecessary cache files from time to time.


To delete unnecessary cache files, using Windows Explorer browse over to the location specified in Tools - Options - File Locations for data cache files. Set Windows Expolorer so it shows file details, including the date each file was last modified and then right click on the Modified column in Windows Explorer to sort the files by the date of their last modification. Delete those files that haven't been used for a long time to free up disk space.


See Also


Linked Images from OGC WMS Servers

Linked Images from TerraServer

Tools - Options - File Locations