Clients starting up from OS X Yosemite diskless images store shadow files on the server.
By default, NetBoot for Mac clients creates a share point for client shadow files on the server boot volume. (You can change this behavior. See Change how NetBoot clients allocate shadow files.)
You can use the Server app to see this share point and to add others. The share points are named NetBootClientsn, where n is the share point number.
Share points are numbered starting with zero. For example, if your server has two disk volumes, the default shadow-file folder is NetBootClients0 on the boot volume. If you use the Server app to specify that client data will also be stored on the second volume, the folder is named NetBootClients1. NetBoot stores the first client’s shadow files on NetBootClients0, the second client’s shadow files on NetBootClients1, the third client’s shadow files on NetBootClients2, and so on.
Likewise, with three volumes and eight clients, the first, fourth, and seventh clients use the first volume; the second, fifth, and eighth clients use the second volume; and the third and sixth clients use the third volume. This load balancing is automatic and usually provides optimal performance.
To prevent shadow files from being placed on a specific volume, use the NetInstall pane of the Server app. Deselect the client data checkbox for any volume you don’t want shadow files placed in.
You can also prevent shadow files from being placed on a specific volume or partition by deleting the hidden file /Library/NetBoot/.clients, which is a symbolic link, then stopping and restarting NetInstall service.