Austin S Hemmelgarn posted on Tue, 18 Aug 2015 07:34:09 -0400 as excerpted: >> 4-device raid6, btrfs and mdraid both allow that, good point. But of >> course mdraid6 doesn't have the data integrity, only rebuild-parity. >> > Huh, I didn't know that mdraid allowed that, I know dm-raid through LVM > doesn't (which in turn is a large part of what caused me to try btrfs > raid56 so soon, I had been going to do btrfs raid1 on top of LVM based > raid6). Yes. I ran a 4-device mdraid-6 for a couple years or so. Then I got tired of the write performance, and the devices were getting old and I found I had room if I squeezed, so I switched to 4-way mdraid1 on the same devices, for awhile. That's actually what I was running when I first looked into btrfs, and was hugely disappointed that btrfs could only do pair-mirrored raid1. The devices were old enough I didn't trust pair-mirrored, and wanted at least 3-way, so I didn't switch to btrfs at that time. I only switched to btrfs sometime later, after I upgraded the core system, and primary storage to ssd. I had been impressed with reiserfs, which I still use on my spinning rust, but decided its journaling wasn't going to be good for ssds, and while I was upgrading to something a bit more ssd friendly, decided I might as well go btrfs. So now I'm running btrfs on the two ssds, partitioned identically, with several btrfs raid1 filesystems on parallel partitions on the two, with primary (fat-finger-level) backup on other partitions on the same devices. Only /boot (and the gpt bios and efi partitions, bios currently used, efi pre-allocated for easy mobo upgrade) is not btrfs raid1. It's btrfs mixed-bg-mode dup, with an independent btrfs on each device, with grub2- core independently installed to the gpt bios partition on each device as well, so I can boot either one by simply selecting it in the bios, allowing the one to be the backup for the other, much like additional partitions and btrfs raid1 on the same physical pair are backups for the working copy of other partitions. But a backup raid1 btrfs won't work for /boot, since grub can really only point at one /boot (tho of course with grub2-core installed in the bios partition, I could use grub emergency mode to select a backup /boot if necessary, but that's harder than simply having two independent /boots setup, one per device), which is why I run two independent dup-mode /boots. Secondary backup is to reiserfs on another device, spinning rust, with third backup to a normally disconnected external USB device. For my use- case, I decided off-site backup isn't worth the hassle, as the backup that /really/ counts is in my head, and if all the on-site backups including the external USB drive are unavailable, chances are a rather large real-life disaster (like my neighbor's house that burned down a few weeks ago) happened, and the things I'll be worried about then will make worrying about a lost computer backup look rather trivial. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- 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
