Reading 2 disks is not very much slower than 1. Its always the I/o bandwidth that is at issue, so it very much depends on the connectivity.
And it all pretty much impacts your customer. We've all suffered under the 4-hour mirror rebuild, the whole machine made inoperable by the constant disk load. The only way to alleviate that, is to design extra bandwidth (e.g. another cable and controller) that's used exclusively for recovery.
I've designed/build storage that worked that way, but it was for enterprise. The home user doesn't want to pay for extra bandwidth and then not use it most of the time. And if the DO use it, then they notice when a rebuild is triggered, and complain. Its catch-22.
>Reading 2 disks is not very much slower than 1. Its always the I/o bandwidth that is at issue, so it very much depends on the connectivity.
This was a problem in the old shared-bus u320 days... but now that we've got a 3 or 6 gigabit serial link to each disk? the bottleneck, unless you have some super-fancy SSD shit going on, is going to be getting the bits off the disk. I don't know of any spinning disk that can consistently saturate even a 2 gigabit link.
That's the thing... random access on spinning rust is staggeringly slow compared to almost everything else your computer does... and while a rebuild is sequential access, mostly, if you are trying to use the system during the rebuild? well, simultaneous sequential accesses become random access, so yea, your system is gonna suck during the rebuild anyhow. Add to this, well, disk diagnostics suck. Quite often a single disk will perform under-spec for some time before failing, slowing down the whole raid.
But SATA solved almost all of the bus bottleneck issues when it comes to disks.
Can you elaborate on the second controller? Are you referring to two disks on two controllers, using mdadm (or similar) to create a software RAID 1 across the two?
And it all pretty much impacts your customer. We've all suffered under the 4-hour mirror rebuild, the whole machine made inoperable by the constant disk load. The only way to alleviate that, is to design extra bandwidth (e.g. another cable and controller) that's used exclusively for recovery.
I've designed/build storage that worked that way, but it was for enterprise. The home user doesn't want to pay for extra bandwidth and then not use it most of the time. And if the DO use it, then they notice when a rebuild is triggered, and complain. Its catch-22.