I always wondered... does the battery power the disk so the writes are done or does it just hold the data in the RAID controller until the computer is re-powered?
On power loss the drives will immediately lose power, they have little capacitors in them that allow the head to safely rest on power loss. (it used to be that the head just crashed into the spinning platter, can you even imagine).
Part of the Server boot-up is to flush the raid caches, which are battery backed volatile storage. (and the batteries are fairly big so can last about 7 days in most cases).
The battery being big doesn't matter to the drives though, there would be no guarantee that you could finish writing the cache to the drives with the capacity of the battery: since drives use a considerably larger amount of power than keeping some DIMMs hot.
> it used to be that the head just crashed into the spinning platter, can you even imagine
If I remember correctly, it was not that bad. The head being parked in a random place meant it could damage the plate if the disk was moved or shaken. I think that modern disks would indeed crash (if not for autoparking) but they have much smaller flying heights and thus require serious active magic to keep the head from hitting the plate.
This is correct - parking the heads moved the head to the side so it wasn't above the platter; but just pulling power meant the head was still above the platter, but no damage.
Parking would mean you could move the drive and the head wouldn't bounce off the platter.
Exactly - the better controllers would "lock" and basically refuse to do anything until they saw the array they wanted to write to (which was always fun when dealing with a remaindered system pulled from production, often requiring a full BIOS reset or wipe on the card).
> On power loss the drives will immediately lose power, they have little capacitors in them that allow the head to safely rest on power loss. (it used to be that the head just crashed into the spinning platter, can you even imagine).
I remember as a kid running PARK.COM to park the heads before powering off the computer every night; I continued this habit for many years after I got my own computer, until I learned that hard drives now have mechanisms that make this unnecessary.
The controllers typically accept writes in a log structure that can be replayed once power is restored. Because the battery is only powering some SRAM (might be more DRAM these days) and a small ASIC or whatever, not even the whole controller, a small-laptop sized battery can keep that up for sometimes days. Lots of variability in how those are implemented depending on how enterprise-y and expensive they are.
