This article is going over what the Solr Asset Cache is and what it is used for.
The Asset Cache folder is used for when the Solr server and CMS server are on separate boxes. When Solr crawls a site's assets, it looks to the CMS server and will pull over the asset information to the Solr server, using the Asset Cache folder as a storage location. This allows for quicker access when an end user queries a title for a PDF, for example.
Where the Asset Cache folder lives, depends on where it was specified during installation. If you wish to move this folder to another drive after Solr has already been installed, Solr will need to be re-installed.
For optimal performance, we recommend a provision of three high-performance spindles as follows.
D-Solr install: Where the installer places the core components, including the crawl database.
E-Solr index/Asset: Selected during installation.
We also recommend, that the 'E' drive in this instance, to have 2.5 times the size of your all of your site's assets (assets, private assets, uploadedfiles/images) to allow for growth.