Assuming that you have many more disks than 3, then the chances that 3 simultaneous disk failures being just the right 3 is much lower than the chances of losing any 3 disks. This is enhanced by the ability of Hadoop to allocate files in different racks since one of the few mechanisms of coordinating failures is losing an entire rack.
For example, if you have 20 disks, then the chance of losing a particular three disks given that you are losing 3 disks is about one chance in a thousand (assuming independent error location) and should be impossible if the failures are rack aligned. Remember, you can always increase the number of replicas if you like. On 7/17/07 12:55 AM, "Phantom" <[EMAIL PROTECTED]> wrote: > Is replica management built into HDFS ? What I mean is if I set replication > factor to 3 and if I lose 3 disks is that data lost forever ? I mean all 3 > disks dying at the same time I know is a far fetched scenario but if they > die over a certain period of time does HDFS re-replicate the data to ensure > that there are always 3 copies in the system ? > > Thanks > A
