With RAID-1, all data is written to two disks. This RAID level is also known as mirroring, or shadowing. Advantages include increased MTBF (mean time between failure) and fast data reads. MTBF increases since the failure of one disk does not stop a system from running--the system can continue to operate with the alternate, mirrored disk. Fast reads are also a benefit because the read can be performed from either disk and the least busy one will be used. Disadvantages include the increased write time (since each write must be duplicated to two disks) and increased cost of purchasing the doubled amount of storage.