Why not RAID-0? It’s about Time and Snowflakes

http://hortonworks.com/blog/why-not-raid-0-its-about-time-and-snowflakes/

Reliability

Before panicking – disk failures are rare. Google’s 2007 paper, Failure Trends in a Large Disk Drive Population, reported that in their datacenters, 1.7% of disks failed in the first year of their life, while three-year-old disks were failing at a rate of 8.6%. About 9% isn’t a good number.（超过三年的硬盘发生问题的概率在9%） 8块超过3年的磁盘同时使用出现问题的概率在1-（1-0.086）^8 = 0.513，这个几率还是相当高的。这个还不是主要的问题，因为JBOD: Just a Box of Disks也会遇到这个问题。

主要问题是，如果一旦一块磁盘出现问题的话，那么所有的磁盘上的数据都需要进行replication.因为RAID0是strip存储的，每个disk上面可能存储一个small block（64KB），而HDFS使用64MB作为block。这就意味着1个HDFS block在10 RAID0 disks上面的话会分摊在10个disk上面，如果一个disk出现问题的话，那么所有的HDFS block都发生损坏就都要进行replication

Every Disk is a Unique Snowflake

• On RAID-0 Storage the disk accesses go at the rate of the slowest disk. RAID0带宽瓶颈限制在slowest disk上面
• The 2011 paper, Disks Are Like Snowflakes: No Two Are Alike, measured the performance of modern disk drives, and discovered that they can vary in data IO rates by 20%, even when they are all writing to same part of the hard disk.
• if you have eight disks, some will be faster than the others, right from day one. And your RAID-0 storage will deliver the performance of the slowest disk right from the day you unpack it from its box and switch it on.