On Fri, Feb 6, 2015 at 1:01 PM, Brian B <canis8585@xxxxxxxxx> wrote: > My laptop has two disks, a SSD and a traditional magnetic disk. I plan > to make a partition on the mag disk equal in size the SSD and set up > BTRFS RAID1. This I know how to do. There isn't a write mostly option in btrfs like there is with md raid, so I don't know how Btrfs will tolerate one device being exceptionally slower than the other. It may be most of the time it won't matter, but I can imagine with a ton of IOPS backing up on the hard drive, having completed on the SSD, could maybe be a problem. I'd test it unless someone else who has pipes up. > > The only reason I'm doing the RAID1 is for the self-healing. I realize > writing large amounts of data will be slower than the SSD alone, but > is it possible to set it up to only read from the magnetic drive if > there's an error reading from the SSD? No. > > In other words, is there a way to tell it to only read from the faster > disk? Is that even necessary? Is there a better way to accomplish > this? No. No. And maybe. In order. If there is an error detected by either drive, or by Btrfs, Btrfs will get the correct data from the other drive and fix the problem on the original drive. You don't need to configure anything. The only concern is the asymmetric performance. I think the use case is better achieved with two HDD's + two SSD partitions, configured either with bcache or dmcache. The result is two logical devices using HDDs as backing device and SSD partitions as cache, and then format them as Btrfs raid1. The question there of course, is maturity of bcache vs dmcache and their interactions with Btrfs. But at least that's supposed to work. -- Chris Murphy -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
