Marc MERLIN posted on Sun, 04 May 2014 18:27:19 -0700 as excerpted: > The original reason why I was asking myself this question and trying to > figure out how much better -m raid1 -d raid0 was over -m raid0 -d raid0 > > I think the summary is that in the first case, you're going to to be > abel to recover all/most small files (think maildir) if you lose one > device, whereas in the 2nd case, with half the metadata missing, your FS > is pretty much fully gone. > Fair to say that?
Yes. =:^) > Now, if I don't care about speed, but wouldn't mind recovering a few > bits should something happen (actually in my case mostly knowing the > state of the filesystem when a drive was lost so that I can see how many > new files showed up since my last backup), it sounds like it wouldn't be > bad to use: > -m raid1 -d linear Well, assuming that by -d linear you meant -d single. Btrfs doesn't call it linear, tho at the data safety level, btrfs single is actually quite comparable to mdadm linear. =:^) (I had to check. I knew I didn't remember btrfs having linear as an option, and hadn't seen any patches float by on the list that would add it, but since I'm not a dev I don't follow patches /that/ closely, and thought I might have missed it. So I thought I better go check to see what this possible new linear option actually was, if indeed I had missed it. Turns out I didn't miss it after all; there's still no linear option that I can see, unless it's there and simply not documented. =:^) > This will not give me the speed boost from raid0 which I don't care > about, it will give me metadata redundancy, and due to linear, there is > a decent chance that half my files are intact on the remaining drive > (depending on their size apparently). Yes. =:^) > So one place I use it is not for speed but for one FS that gives me more > space without redundancy (rotating buffer streaming video from security > cams). > At the time I used -m raid1 -d raid0, but it sounds for slightly extra > recoverability, I should have ued -m raid1 -d linear (and yes, I > undertand that one should not consider a -d linear recoverable when a > drive went missing). That appears to be a very good use of either -d raid0 or -d single, yes. And since you're apparently not streaming such high resolution video that you NEED the raid0, single does indeed give you a somewhat better chance at recovery. Tho with streaming video I wonder what your filesizes are as video files tend to be pretty big. If they're over the 1 GiB btrfs data chunk size, particularly if you're only running a two-device btrfs, you'd probably lose near all files anyway. Assuming single data mode and file sizes between a GiB and 2 GiB, statistically you should lose near 100% on a two device btrfs with one dropping out, 67% on a three device btrfs with a single device dropout, 50% on four devices, 40% on five devices... If file sizes are 2-3 GiB, you should lose near 100% on 2-3 devices, 75% on four devices, 60% on five, 50% on six... With raid0 data stats would be similar but I believe starting at 16 MiB with 4 MiB intervals. Due to many files under 16 MiB being stored in the metadata, you'd lose few of them, but that'd jump to 100% loss at 16 MiB until you had 5+ devices in the raid0, with 16-20 MiB file loss chance on a 5-device raid0 80%, since chances would be 80% of one strip of the stripe being on the lost device. (That's assuming my 4 MiB strip size assumption is correct, it could be smaller than that, possibly 64 KiB.) -- 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 majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
