Hi All Try to gain some idea here. I got very different dd result on write to a whole disk and a partition in the disk. i intentionally write small data, assumed they should go to page cache and return very fast. but not. # dd if=/dev/zero of=/dev/sdq1 bs=1M count=4 seek=2 4+0 records in 4+0 records out 4194304 bytes (4.2 MB) copied, 0.262441 seconds, 16.0 MB/s # dd if=/dev/zero of=/dev/sdq bs=1M count=4 seek=2 4+0 records in 4+0 records out 4194304 bytes (4.2 MB) copied, 0.136515 seconds, 30.7 MB/s so writing to whole disk is 2 time faster than writing to a partition. Kernel is 2.6.22.15, reboot to RHEL5 kernel, same result. fdisk shows. Disk /dev/sdq: 430.2 GB, 430244364288 bytes 255 heads, 63 sectors/track, 52307 cylinders, total 840321024 sectors Units = sectors of 1 * 512 = 512 bytes Device Boot Start End Blocks Id System /dev/sdq1 128 840311954 420155913+ 83 Linux by using blktrace, i found that writing to whole disk /dev/sdq almost finish immediately and blktrace show nothing, until couple seconds later that show io with 8 sector in size and process is pdflush. but write to partition then blktrace show data immediately, in dd process context. and most data show like 65,0 1 8238 0.008146979 6006 Q W 12325 + 2 [dd] 65,0 1 8239 0.008147222 6006 M W 12325 + 2 [dd] 65,0 1 8240 0.008147741 6006 Q W 12327 + 2 [dd] 65,0 1 8241 0.008147975 6006 M W 12327 + 2 [dd] 65,0 1 8242 0.008148485 6006 Q W 12329 + 2 [dd] 65,0 1 8243 0.008148758 6006 M W 12329 + 2 [dd] 65,0 1 8244 0.008149271 6006 Q W 12331 + 2 [dd] no idea why here the request size is so small... try to check the call path and see where limit the size, but did not have any luck. then by adjusting the partition starting offset, i see different request size here range from +1 to +2 to +8.... but no matter what, all go inline in dd process context. so why starting offset make such big difference? one would assume that make partition align to power of 2 will make it faster, but ironically make it odd will get +8 or +4 while even offset always get +1. another big surprise, by blktracing a partition, io becomes "faster"... here is ./blktrace -d /dev/sdq -o - |./blkparse -i - [root@ks02 ~]# dd if=/dev/zero of=/dev/sdq bs=1M count=4 seek=2 4+0 records in 4+0 records out 4194304 bytes (4.2 MB) copied, 0.00300966 seconds, 1.4 GB/s which make writing to whole disk faster but to partition unchanged. but when using ./blktrace -d /dev/sdq1 -o - |./blkparse -i - write to partition is faster too. [root@ks02 ~]# dd if=/dev/zero of=/dev/sdq1 bs=1M count=4 seek=2 4+0 records in 4+0 records out 4194304 bytes (4.2 MB) copied, 0.00397497 seconds, 1.1 GB/s also changing scheduler does not matter... so by using blktrace, page cache is enabled... any idea? if need more data or test run, let me know -- Ming Zhang @#$%^ purging memory... (*!% http://blackmagic02881.wordpress.com/ http://www.linkedin.com/in/blackmagic02881 -------------------------------------------- - To unsubscribe from this list: send the line "unsubscribe linux-btrace" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
