On Sat, Nov 22, 2014 at 8:22 AM, Anshuman Aggarwal <anshuman.aggarwal@gmail.com> wrote:
By not using stripes, we restrict writes to happen to just 1 drive and the XOR output to the parity drive which then explains the delayed and batched checksum (resulting in fewer writes to the parity drive). The intention is that if a drive fails then maybe we lose 1 or 2 movies but the rest is restorable from parity.
Also another advantage over RAID5 or RAID6 is that in the event of multiple drive failure we only lose the content on the failed drive not the whole cluster/RAID.
Did I clarify better this time around?
I still don't understand the delayed checksum/parity. With classic raid 4, writing 1 GB of data to just D1 would require 1 GB of data first be read from D1 and 1 GB read from P then 1 GB written to both D1 and P. 4 GB worth of I/O total. With your proposal, if you stream 1 GB of data to a file on D1: - Does the old/previous data on D1 have to be read? - How much data goes to the parity drive? - Does the old data on the parity drive have to be read? - Why does delaying it reduce that volume compared to Raid 4? - In the event drive 1 fails, can its content be re-created from the other drives? Greg -- Greg Freemyer