Chris Murphy posted on Tue, 22 Apr 2014 11:42:09 -0600 as excerpted:
> On Apr 21, 2014, at 3:09 PM, Duncan <[email protected]> wrote: > >> Adam Brenner posted on Sun, 20 Apr 2014 21:56:10 -0700 as excerpted: >> >>> So ... BTRFS at this point in time, does not actually "stripe" the >>> data across N number of devices/blocks for aggregated performance >>> increase (both read and write)? >> >> What Chris says is correct, but just in case it's unclear as written, >> let me try a reworded version, perhaps addressing a few uncaught >> details in the process. > > Another likely problem is terminology. It's 2014 and still we don't have > consistency in basic RAID terminology. > It's not immediately obvious to the btrfs newcomer that the md raid > chunk isn't the same thing as the btrfs chunk, for example. > > And strip, chunk, stripe unit, and stripe size get used interchangeably > to mean the same thing, while just as often stripe size means something > different. FWIW, I did hesitate at one point, then used "stripe" for what I guess should have been strip or stripe-unit, after considering and rejecting "chunk" as already in use. But in any case, while btrfs single mode is distinct from btrfs raid0 mode, and because the minimum single-mode unit is 1 GiB and thus too large to do practical raid0, on multiple devices btrfs single mode does in fact end up in a sort of raid0 layout, just with too big a "strip" to work as raid0 in practice. IOW, btrfs single mode layout is one 1 GiB chunk on one device at a time, but btrfs will alternate devices with those 1 GiB chunks (choosing the one with the least usage from those available), *NOT* use one device until it's full, then another until its full, etc, like md/raid linear mode does. In that way, the layout is raid0-like, even if the chunks are too big to be practical raid0. Btrfs raid0 mode, however, *DOES* work as raid0 in practice. It still allocates 1 GiB chunks per devices, but does so in parallel across all available devices, and then stripes at a unit far smaller than the 1 GiB chunk, using I believe a 64 or 128 KiB strip/stripe-unit/whatever, with the full stripe-size thus being that times the number of devices in parallel in the stripe. <sigh> It's all clear in my head, anyway! =:^( -- 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 [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
