Hello, I am trying to understand what differences there are in using btrfs raid1 vs raid10 in terms of recoverability and also performance. This has proven itself to be more difficult than expected since all search results I could come up with generally suffer from one of three flaws: they either discuss terribly old versions of btrfs, only discuss 4 disk settings, or are about traditional HW (or mdadm) RAID modes.
>From what I gathered so far, with raid1 btrfs just puts the 2 copies of a file on 2 different devices. And raid10 splits files into stripes, then writes 2 copies of each stripe to 2 different devices. By splitting the files into stripes it can write stripe 1 to devices A and B, while at the same time writing stripe 2 to devices C and D, and so on. So a single copy of a file might end up split across all devices, as does the second, but with the stripes distributed in a way that the copies of each one stripe are never on the same device. So my first question is: is that actually correct? Or does btrfs raid1 create copies of blocks or something akin to stripes instead of files? Because I imagine if it is at the file level there is a difference in recoverability if the "wrong" 2 devices die. For a raid1 I'd expect to only loose those files whose copies were located on those 2 devices. Every file with a copy on one of the still working devices would be recoverable. So the more devices there are the bigger the percentage of recoverable files could get. While with raid10 the copies of every file's first stripe might end up on device A and device B, damaging every single file if A and B die at the same time. This might just be a reason for me to choose raid1 over raid10, so I really appreciate if someone could enlighten me ;) As to performance, with raid1 write speed should (theoretically) be the same as a single disk (although writing the first half of the data to device A while at the same time writing the second half to device B would allow to write the first copy in half the time, and would allow to create the second copy at some later point in time I highly doubt btrfs is quite that adventurous). And read speeds should be up to twice that of a single device. With raid10 write speeds should be N times those of a single disk to create the first copy, and since of course a second one has to be written as well, effectively up to N/2. Read speeds should be up to N times that of a single disk. But I couldn't find useful comparisons using more than 4 devices. Should I expect any weirdness if I don't have a multiple of 4 devices? Or do I just need an even number of devices? Or is everything ok, even odd numbers? And finally, could using raid10 cause me more headache than raid1 farther down the line when adding additional devices? How about if those devices are not the same size as the original ones, any difference between raid1 and 10? Thank you for your help! Alexander -- 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
