Having a bit of writers block this morning (“write blog article” appeared in Outlook calendar but brain is not responding in kind), so thought I’d take the easy way out and give a quick technical update on our products with some obscure, bizzare or just not-widely-known bit of information.
“Auto Rebuild” is an option in the bios of our cards about which most people have no idea how it works. This could have been called “the option that helps the forgetful system admin” or “even if you don’t bother we’ll safeguard your data for you if we can” … but I don’t think either of those descriptions would fit I the bios screen. “Auto Rebuild” is so last century (but that’s OK because it’s been around for probably that long).
So what does it do? When enabled (which it is by default) … when the card finds an array that is degraded it will first look for a hot spare. If this is not found then it will look for any unused devices (drives that are not part of an array). If a suitable unused disk is found (right size), then the card will build that disk back into the array.
Why did we need this? We have lots of hot spare options – why bother with an auto feature as well? My theory on this (and I’ll probably never know the full exact reason) is that system builders often send off a new system with a hot spare in place, but when the drive fails the user/customer will replace the drive, but does not know how to, or that they should, or that they need to, make that new drive a hot spare. So in my experience the “new drive” that has replaced the old failed drive, is often just sitting as a raw device … and when the next drive fails it does nothing because it’s not a “hot spare”.
Well in the case of our cards set to their factory defaults … the new drive will kick in and replace a failed drive (and rebuild the array), because of “auto rebuild” or “even if you don’t bother we’ll safeguard your data for you if we can” (as I like to call it).
Did you know that?
Quick update. I might have generalised a bit too much on this one. Note that this works in a ses2 backplane (hot swap backplane) when you change the drive in the same slot. Note also that in a series 7 controller it works if you change the new drive into the same slot as the old drive, but if you put the new drive into a different slot you’ll have to make it a hot spare because an S7 will grab that drive and show it to the OS as a pass-through device. Was trying to keep this simple, but probably went a bit far. Oh the complexities we weave