2012/9/14 Theodore Ts'o <tytso@xxxxxxx>: > > The reason for this is how meta_bg changes how the block group > descriptors are laid out. Originally, the block group descriptors > were located contiguously. From a 12T filesystem without meta_bg, > you'll see this from dumpe2fs: > > Group 0: (Blocks 0-32767) > Primary superblock at 0, Group descriptors at 1-768 > > If the file system is created with meta_bg, then group descriptors > that have to be read when the file system is opened by libext2fs or > when the file system is mounted look like this: > > Group 0: (Blocks 0-32767) > Primary superblock at 0, Group descriptor at 1 > Group 128: (Blocks 4194304-4227071) [INODE_UNINIT] > Group descriptor at 4194304 > Group 256: (Blocks 8388608-8421375) [INODE_UNINIT] > Group descriptor at 8388608 > Group 384: (Blocks 12582912-12615679) [INODE_UNINIT] > Group descriptor at 12582912 > ... > > In the set of kernel and e2fsprogs patches that I just released, we > can partially work around this problem by starting with the > resize_inode, and only switch over to the meta_bg once we have > exhausted the resize_inode scheme. So now we can do this: > > mke2fs -t ext4 -q -O 64bit /dev/vdc 12T > mount /dev/vdc > resize2fs /dev/vdc 18T > > After the resize2fs, the block group descriptors for the first 16TB > will be contiguous: > > Group 0: (Blocks 0-32767) [ITABLE_ZEROED] > Primary superblock at 0, Group descriptors at 1-2048 > > after that, there will be singleton block group descriptor blocks, i.e.: > > Group 131136: (Blocks 4297064448-4297097215) [INODE_UNINIT] > Group descriptor at 4297064448 > > The other thing we can do to speed up the mount times is change how > the kernel to lazily read the block group descriptors, instead of > trying to read them all at mount time, at least once they are no > longer contiguous. I'll look into seeing what we can do to improve > things on that front. > > Regards, > > - Ted Ted, thanks a lot for the detailed explanation. It is very clear. I will find time to test your new patches.. Regards, Kevin Liao -- 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
