2012/1/21 Andreas Dilger <adilger@xxxxxxxxx>: > On 2012-01-20, at 4:20 AM, Bluflonalgul wrote: >> Some (misleading?) article on kernelnewbies.org said the new Ext4 >> Bigalloc feature was about supporting block size up to 1MB. >> I tried to use this feature to read an ext3 fs with 16k blocksize made >> on a Linux Debian Sparc (NAS). >> But I couldn't read such filesystem with the Linux 3.2 kernel on x86 >> PC... It fails to read fs structure (as it used to fail with previous >> kernels). > > The bigalloc feature is not intended to be disk compatible with a > large blocksize filesystem, or no "feature" would be needed at all > besides increasing the blocksize in the superblock. > > What it is intended to handle is efficient block allocation for large > file IO, by increasing the size of space allocation/tracking in the > block bitmap, without breaking the kernel paradigm of keeping block > size <= PAGE_SIZE. > > This gives many of the benefits of having a large blocksize without > needing to change the whole kernel. > >> Could someone point me to some documentation, or give me some clues: >> I'd like to understand what's wrong and if I can hope to read such fs >> with Linux on x86 (natively, without fusefs tricks or additional >> tools). > > There was some work done by Robin Dong (2011-11-18) that would get us > most of the way to just handling large blocksize filesystems directly > by the kernel. This might be facilitated by denying mmap access to > such filesystems, but for media/big data filesystems (as opposed to the > root fs) this is probably not a serious limitation. > > I'm still interested to see a continuation of Robin's work, taking it > to just be disk compatible with large blocksize, even if it is not > possible to use mmap IO on such filesystems (always setting MNT_NOEXEC > on systems where PAGE_SIZE < blocksize and not supplying f_op->mmap > should work). A great idea of extended version of ext4_extent was mentioned by Ted (2011-11-19 http://www.spinics.net/lists/linux-ext4/msg28999.html) I am happily to buy this story which might solve the concerns of TAOBAO corp and Andreas as well. Therefore I will send a RFC later to continue :) > > The reason that this is desirable is that it allows bypassing the 16TB > file size limitations, and it also allows mounting filesystems from > SPARC, PPC, and IA64 systems that were formatted in this manner and are > getting old and need replacing. > > 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 -- -- Best Regard Robin Dong -- 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
