Hi Neil, This is raid5. I have mounted /dev/md0 to /mnt and file system is ext4. The system is newly created. Steps: mdadm for raid5 mkfs.ext4 /dev/md0 mount /dev/md0 /mnt/raid Export /mnt/raid to remote PC using CIFS Copy file from PC to the mounted drive An update.... I just ran the test again (without doing reformatting device) and noticed all 4 HDDs incremented the #ofWritesBlocks equally. This implies that when raid was configured first time, raid5 was trying to do its own stuff (recovery)... What I'm not sure of is if the device is newly formatted, would raid recovery happen? What else could explain difference in the first run of IO benchmark? Thanks. On Fri, Apr 8, 2011 at 4:46 PM, NeilBrown <neilb@xxxxxxx> wrote: > On Fri, 8 Apr 2011 12:55:39 -0700 Linux Raid Study > <linuxraid.study@xxxxxxxxx> wrote: > >> Hello, >> >> I have a raid device /dev/md0 based on 4 devices sd[abcd]. > > Would this be raid0? raid1? raid5? raid6? raid10? > It could make a difference. > >> >> When I write 4GB to /dev/md0, I see following output from iostat... > > Are you writing directly to the /dev/md0, or to a filesystem mounted > from /dev/md0? ÂIt might be easier to explain in the second case, but you > text suggests the first case. > >> >> Ques: >> Shouldn't I see write/sec to be same for all four drives? Why does >> /dev/sdd always have higher value for ÂBlksWrtn/sec? >> My strip size is 1MB. >> >> thanks for any pointers... >> >> avg-cpu: Â%user  %nice %system %iowait Â%steal  %idle >>      Â0.02  Â0.00  Â0.34  Â0.03  Â0.00  99.61 >> >> Device:      Âtps  Blk_read/s  Blk_wrtn/s  Blk_read  Blk_wrtn >> sda        1.08    247.77    338.73  37478883  51237136 >> sda1       Â1.08    247.77    338.73  37478195  51237136 >> sdb        1.08    247.73    338.78  37472990  51245712 >> sdb1       Â1.08    247.73    338.78  37472302  51245712 >> sdc        1.10    247.82    338.66  37486670  51226640 >> sdc1       Â1.10    247.82    338.66  37485982  51226640 >> sdd        1.09    118.46    467.97  17918510  70786576 >> sdd1       Â1.09    118.45    467.97  17917822  70786576 >> md0       Â65.60    443.79   Â1002.42  67129812 Â151629440 > > Doing the sums, for every 2 blocks written to md0 we see 3 blocks written to > some underlying device. ÂThat doesn't make much sense for a 4 drive array. > If we assume that the extra writes to sdd were from some other source, then > It is closer to a 3:4 ratio which suggests raid5. > So I'm guessing that the array is newly created and is recovering the data on > sdd1 at the same time as you are doing the IO test. > This would agree with the observation that sd[abc] see a lot more reads than > sdd. > > I'll let you figure out the tps number.... do the math to find out the > average blk/t number for each device. > > NeilBrown > > >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-raid" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at Âhttp://vger.kernel.org/majordomo-info.html > > -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
