https://bugzilla.kernel.org/show_bug.cgi?id=54271 --- Comment #7 from Phillip Susi <psusi@xxxxxxxxxx> 2013-02-26 15:57:55 --- dd if=/dev/zero of=foo bs=1MiB count=512 echo 1 > /proc/sys/vm_drop_caches time readahead foo && iostat -d 2 real 0m0.212s user 0m0.000s sys 0m0.164s Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn sda 13.07 348.27 564.72 24916849 40401892 sdb 12.34 342.80 564.88 24525467 40413276 sdc 12.64 353.08 558.45 25260487 39953612 sdd 12.70 344.31 561.36 24633134 40162088 md0 128.52 690.00 1518.61 49365215 108647060 dm-0 4.09 68.50 14.09 4900585 1008368 dm-1 1.13 529.16 10.01 37857989 716364 dm-2 0.00 0.01 0.00 748 0 dm-3 104.29 16.60 1494.47 1187561 106920224 Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn sda 83.50 42244.00 0.50 84488 1 sdb 73.50 37122.00 0.50 74244 1 sdc 76.50 38658.00 0.50 77316 1 sdd 80.00 40450.00 0.50 80900 1 md0 1.00 2.00 2.00 4 4 dm-0 1.00 2.00 2.00 4 4 dm-1 0.00 0.00 0.00 0 0 dm-2 0.00 0.00 0.00 0 0 dm-3 0.00 0.00 0.00 0 0 Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn sda 33.00 16384.00 2.50 32768 5 sdb 40.00 20224.00 0.50 40448 1 sdc 37.00 18688.00 0.50 37376 1 sdd 32.00 15872.00 2.50 31744 5 md0 0.00 0.00 0.00 0 0 dm-0 0.00 0.00 0.00 0 0 dm-1 0.00 0.00 0.00 0 0 dm-2 0.00 0.00 0.00 0 0 dm-3 0.00 0.00 0.00 0 0 Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn sda 1.50 0.00 1.00 0 2 sdb 2.50 0.00 19.00 0 38 sdc 1.50 0.00 1.00 0 2 sdd 2.50 0.00 19.00 0 38 md0 5.00 0.00 18.00 0 36 dm-0 4.50 0.00 18.00 0 36 dm-1 0.00 0.00 0.00 0 0 dm-2 0.00 0.00 0.00 0 0 dm-3 0.00 0.00 0.00 0 0 When I was seeing the blocking, I observed the following stat output from a stat command on the file in a debugfs session: EXTENTS: (ETB0):33795, (0-30975):370688-401663, (30976-40191):401664-410879, (ETB0):483328, (40192-72575):410880-443263, (72576-81919):443264-452607, (ETB0):483329, (81920-111578):452608-482266 As you can see, the file has 3 level zero extent tree blocks. That last extent of blocks (452608-482266) can't be queued for reading until the ETB at 483329 has been read, and so the readahead blocks until all of the blocks before have been read, because they are ahead of the ETB in the queue, then once it has the ETB, it queues the last extent and returns. The freshly created file with dd has no extent tree blocks to read, so no blocking happens. -- Configure bugmail: https://bugzilla.kernel.org/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- You are watching the assignee of the bug. -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
