RAID 5 has a well-known flaw called the RAID 5 write hole. This causes a problem when a data block is written to a stripe but a power failure occurs before the corresponding parity block can be written. As a result, the data and parity for the stripe will be inconsistent. If a disk then fails, the RAID reconstruction process will result in incorrect data. The only way out of this is if an entire stripe happens to be overwritten, thus generating a correct parity block. RAIDZ gets around this problem by using a variable width stripe, so every write is effectively a full stripe write. This, together with ZFS’s copy on write characteristic, eliminates the RAID 5 write hole completely. RAIDZ2 works in a similar way, but can tolerate the loss of two disks in the array using double parity.