SVR is nothing but a DC-DC PCB.
These SVR\'s will be installed in the HDU boxes and also includes the HDD status LED\'s. (If my memory is correct the number of SVR PCB\'s is 9 per HDU box and 1 will be spare). This will be always on and give power to 8 HDD\'s. If more than two SVR fail, SVR cannot give power for more than 8 HDD\'s. If two failed SVR covers more than 8 HDD\'s, then all HDD\'s stop working. for example if the 2 failed SVR cover 16 HDDs then all 16 HDD\'s will stop working.
Also if the 2 failed SVR covers 7 HDDs, then the spare SVR will give power to these 7 HDDs.