Revisiting Raid 5 And Wide Stripe Or RAID Groups

Let us begin with an example from part one in this series of a 15+1 or sixteen-drive RAID 5 group. For some applications and RAID 5 (or RAID 4 or RAID 6) implementations, a 15+1 (or wider) stripe or group might be sufficient.

However, writes could become a bottleneck particular if there is no mirrored or battery protected write back cache (WBC). Another common myth comes into play that all RAID 5 implementations cause extra write IO activity. While some RAID implementations in hardware or software can result in extra back-end writes (e.g. write amplification), this is not true for all, particularly those with good write gathering capabilities.

Some hardware and software implementations using WBC (mirrored or battery backed-BBU) can group writes together in memory (cache) to do full stripe writes. The result can be fewer back-end writes compared to other systems. Hence, not all RAID implementations in either hardware or software are the same. Likewise, just because a RAID definition shows a particular theoretical implementation approach does not mean all vendors have implemented it in that way.

RAID: Extra Writes?

So, does RAID cause extra writes or write amplification?

That depends on the particular RAID level, along with implementation and, in some cases, configuration, including chunk or shard size. For example, RAID 1 (mirroring and replication) does two or more writes in parallel, which would be the same as copying to two disk drives. Granted some implementations may do the writes in real-time (synchronous) or time-deferred (asynchronous, lazy write, or eventual consistency) modes in addition to leveraging WBC.

Do the dual writes of a RAID 1 implementation mean that there are double the number of writes (or triple with three drive mirrors)?