On Fri, Sep 19, 2014 at 11:54:39AM -0400, TR Reardon wrote: > Hello all: there's probably a good reason for this, but I'm wondering why inode#7 (reserved GDT blocks) is always created with a block map rather than extent? > > [see ext2fs_create_resize_inode()] It's created using an indirect map because the on-line resizing code in the kernel relies on it. It's rather dependent on the structure of the indirect block map so that the kernel knows where to fetch the necessary blocks in each block group to extend the block group descriptor. So no, we can't change it. And we do have a solution, namely the meta_bg layout which mostly solves the problem, although at the cost of slowing down the mount time. But that may be moot, since one of the things that I've been considering is to stop pinning the block group descriptors in memory, and just start reading in memory as they are needed. The rationale is that for a 4TB disk, we're burning 8 MB of memory. And if you have two dozen disks attached to your system, then you're burning 192 megabytes of memory, which starts to fairly significant amounts of memory, especially for bookcase NAS servers. If I would do it all over again, knowing what we know now, I'd probably redesign the meta_bg layout somewhat to group block group descriptors into chunks. But it's probably not worth it to add yet another block group descriptor layout at this point. Cheers, - Ted -- 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
