XDP isn't exactly 23+2, but it's a good simple way to think of it.  The dual parity (+2) means that up to 2 disks can fail at a time - that can mean both fail simultaneously, or that one fails and then the second fails before the rebuild of the first one completes.  Where XDP is very different to RAID is that when a disk fails, it doesn't hot-spare to a dedicated disk, but instead actually re-layouts the data to change the stripe size.  So if you're normally operating at 23+2, and one disk fails, then after the rebuild has been completed it'll be a healthy 22+2 layout.  If another disk fails, it'll become 21+2.  If two more fail simultaneously then after the rebuild it'll be 19+2, and so on - down to a minimum of 18+2 (thus your 5 disks that can fail and still be healthy)  Of course, all of this presumes that there's enough free space on the array to handle the loss of the disks.  The first failure has reserved space available (spread out across all of the disks in the X-Brick), so there's no impact to available space.  Each subsequent failure will reduce the available space on the array to allow for the hot-spare-space.  In practice, this is all very much academic. SSDs fail far less frequently than spinning disks - the drives in XtremIO have a MTBF of 2 million hours (around 228 years), and we're actually seeing them fail even less frequently than that!  The odds of a single disk failure are low - the odds of a dual disk failure are minuscule.
Hi everyone,  I read the XDP whitepaper and have a question about the dual drive failure scenario.  Because XDP is 23+2, it seems that we cannot have a dual drive failure simultaneously without data loss.  However, it goes on to say we can have no impact for up to 5 failed SSDs per xbrick so I am a little confused.  Does the dual drive failure mean it happens at the same exact moment? Or does it refer to a single SSD failing and the second one failing while the first is rebuilding ?  Also, what is meant by 5 failed SSDs per xbrick and no impact? This seems to contradict the statement that we cannot sustain dual drive failure simultaneously.
XDP simultaneous dual drive failure question