Hi: I'm seeing a performance problem on ext4 vs ext2, and in trying to narrow it down, I've got a question about block allocation in ext4 that I'm having trouble figuring out. The test in question just does random reads of several rather large files (4.5GB and 10GB) in a single thread. All files are created in the top-level directory. Looking into the block layout for the various files, I'm struck by the wide separation of the extents in some of the files. As a simple example, I formatted/mounted a new ext4 partition with default parameters (with the exception of "-O ^has_journal", but this shouldn't make a difference); the FS has 5585 block groups of 4K blocks. Using dd, I created (in this order) two 4GB files and a 10GB file in the mount directory. The extent blocks are reasonably close together for the two 4GB files, but the extents for the 10GB file show a huge gap, which seems to hurt the random read performance pretty substantially. Here's the output from debugfs: BLOCKS: (IND):8396832, (0-106495):8282112-8388607, (106496-399359):11241472-11534335, (399360-888831):20482048-20971519, (888832-1116159):23889920-24117247, (1116160-1277951):71665664- 71827455, (1277952-1767423):78678016-79167487, (1767424-2125823):102402048-102760447, (2125824-2148351):102768672-102791199, (2148352-2621439):102793216-103266303 TOTAL: 2621441 Note the gap between blocks 79167487 and 102402048. I was lucky enough to capture the mb_history from this 10GB create: 29109 14 735/30720/32758@1114112 735/30720/2048@1114112 735/30720/2048@1114112 1 0 0 1568 M 0 0 29109 14 736/0/32758@1116160 736/0/2048@1116160 2187/2048/2048@1116160 1 1 0 1568 0 0 29109 14 2187/4096/32758@1118208 2187/4096/2048@1118208 2187/4096/2048@1118208 1 0 0 1568 M 2048 4096 I've been staring at ext4_mb_regular_allocator() trying to understand why an allocation with a goal block of 736 ends up with a best found extent group of 2187, and I'm stuck -- at least without a lot of printk messages. It seems to me that we just cycle through the block groups starting with the goal group until we find a group that fits. Again, according to dumpe2fs, block groups 737, 738, 739, ... all have 32768 free blocks. So why we end up with a best fit group of 2187 is a mystery to me. Can anybody give me an insight to what's happening here? Thanks, Curt -- 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
