Comment 4 for bug 645904

In a non-ZEO setup you will have a direct file storage under var/filestorage/Data.fs and blobs stored under var/blobstorage. The blobstorage can have various layouts as specified by a .layout file in it, by default using the "bushy" layout.

If you move to a ZEO setup, you will need to decide wether to use shared-blobs or use a blob zeocache. By default shared-blobs are disabled and thus a zeocache layout is used. The zeocache stores files differently, as it's optimized to hold only part of the data and not mirror the entire blob set.

If you simply update things in place, you might configure the var/blobstorage directory to be both the real blobstorage for the ZEO server and a zeocache for the instances. If you are lucky you only get a warning like you did. If you are unlucky you might overwrite all your actual blobs.

If I have to use ZEO without shared-blobs, I tend to specify the "blob-storage" option on the zope2instance part as "blob-storage = ${buildout:directory}/var/blobcache" to make its purpose clear.