On 2011-02-25, at 12:04 PM, Ted Ts'o wrote: > On Fri, Feb 25, 2011 at 08:05:43PM +0200, Amir Goldstein wrote: >> >> I like your design. very KISS indeed. >> I am just wondering why should BIGALLOC be INCOMPAT and not RO_COMPAT? >> After all, ro mount doesn't allocate and RO_COMPAT features are so muc >> nicer... > > I can try to make it be RO_COMPAT, but one thing my design changes is > that a block group will contain 32768 allocation blocks; so assuming a > 4k blocks, instead of a block group containing a maximum of 32,768 4k > blocks comprising 128 MB, a block group would now contain 32,768 1M > blocks, or 32 GiB, or 8,388,608 4k blocks. > > I'm pretty sure that existing kernels have superblock sanity checks > that will barf if they see this. Still, yeah, I can try allocating > this as a ROCOMPAT feature, and later on, if people really care, they > can patch older kernels so they won't freak out when they see a > BigAlloc file system and can thus successfully mount it read-only. > > (Right now existing kernels will complain when s_blocks_per_group is > greater than blocksize*8.) Hmm, if we stuck with a flex_bg factor G >= the allocation blocksize (2^G * blocksize), then it would appear that the main difference is (2^G - 1) unused block bitmaps per flex_bg, and the single used block bitmap per group would be compressed 2^G:1 (i.e. it wouldn't represent a valid block bitmap to unaware kernels). This would be OK for ROCOMPAT, like Amir wrote. I guess the main difference is that there would still be 2^G more group descriptors to read/scan/write, though they would all be contiguous on disk. To convert away from this feature on an old kernel would mean expanding each bit in flex_bg bitmap[0] by a factor 2^G and clearing the ROCOMPAT flag. Cheers, Andreas -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
