Hi, when I run "mkfs.ext4 /dev/sdc2" in a Linux virtual machine on Hyper-V, where a disk IOPS=500 limit is applied by me [0], the command takes much more time, if the ext4 partition boundary is not properly aligned: Example 1 [1]: it takes ~7 minutes with average wMB/s = 0.3 (slow) Example 2 [2]: it takes ~3.5 minutes with average wMB/s = 0.6 (slow) Example 3 [3]: it takes ~0.5 minute with average wMB/s = 4 (expected) strace shows the mkfs.ext3 program calls seek()/write() a lot and most of the writes use 32KB buffers (this should be big enough), and the program only invokes fsync() once, after it issues all the writes -- the fsync() takes >99% of the time. By logging SCSI commands, the SCSI Write(10) command is used here for the userspace 32KB write: in example 1, *each* command writes 1 or 2 sectors only (1 sector = 512 bytes); in example 2, *each* command writes 2 or 4 sectors only; in example 3, each command writes 1024 sectors. It looks the kernel block I/O layer can somehow split big user-space buffers into really small write requests (1, 2, and 4 sectors)? This looks really strange to me. Note: in my test, this strange issue happens to 4.4 and the mainline 4.9 kernels, but the stable 3.18.45 kernel doesn't have the issue, i.e. all the 3 above test examples can finish in ~0.5 minute. Any comment? Thanks! -- Dexuan [0] The max IOPS are measured in 8KB increments, meaning the max throughput is 8KB * 500 = 4000KB. [1] This is the partition info of my 20GB disk: # fdisk -l /dev/sdc Disk /dev/sdc: 20 GiB, 21474836480 bytes, 41943040 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 4096 bytes I/O size (minimum/optimal): 4096 bytes / 4096 bytes Disklabel type: dos Disk identifier: 0x00000000 Device Boot Start End Sectors Size Id Type /dev/sdc1 1 14281784 14281784 6.8G 82 Linux swap / Solaris /dev/sdc2 14281785 41929649 27647865 13.2G 83 Linux Here, start_sector = 14281785, end_sector = 41929649. [2] start_sector = 14282752, end_sector = 41929649 [3] start_sector = 14282752, end_sector = 41943039 -- 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
