Dave T posted on Wed, 10 Aug 2016 18:01:44 -0400 as excerpted: > Does anyone have any thoughts about using dup mode for metadata on a > Samsung 950 Pro (or any NVMe drive)?
The biggest problem with dup on ssds is that some ssds (particularly the ones with the sandforce controllers) do dedup, so you'd be having btrfs do dup while the filesystem dedups, to no effect except more cpu and device processing! (The other argument for single on ssd that I've seen is that because the FTL ultimately places the data, and because both copies are written at the same time, there's a good chance that the FTL will write them into the same erase block and area, and a defect in one will likely be a defect in the other as well. That may or may not be, I'm not qualified to say, but as explained below, I do choose to take my chances on that and thus do run dup on ssd.) So as long as the SSD doesn't have a deduping FTL, I'd suggest dup for metadata on ssd does make sense. Data... not so sure on, but certainly metadata, because one bad block of metadata can be many messed up files. On my ssds here, which I know don't do dedup, most of my btrfs are raid1 on the pair of ssds. However, /boot is different since I can't really point grub at two different /boots, so I have my working /boot on one device, with the backup /boot on the other, and the grub on each one pointed at its respective /boot, so I can select working or backup /boot from the BIOS and it'll just work. Since /boot is so small, it's mixed- mode chunks, meaning data and metadata are mixed together and the redundancy mode applies to both at once instead of each separately. And I chose dup, so it's dup for both data and metadata. Works fine, dup for both data and metadata on non-deduping ssds, but of course that means data takes double the space since there's two copies of it, and that gets kind of expensive on ssd, if it's more than the fraction of a GiB that's /boot. -- 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
