I have a freshly installed Oracle Linux 7.1 ( akin to RHEL ) server.
However after installing some Oracle software, I have noticed that my hard disk light is continually on and the system performance is slow.
So I check out SAR and IOSTAT
lab3:/root>iostat
Linux 3.8.13-55.1.6.el7uek.x86_64 (lab3) 01/12/15 _x86_64_ (2 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
21.33 0.00 2.66 41.71 0.00 34.30
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 100.94 1928.77 653.97 110966874 37624674
sdb 53.08 929.61 3510.79 53482763 201984646
dm-0 438.70 1351.24 653.67 77740019 37607217
dm-1 0.01 0.02 0.00 1396 0
dm-2 4.93 577.01 0.27 33196938 15409
lab3:/root>sar 5 5
Linux 3.8.13-55.1.6.el7uek.x86_64 (lab3) 01/12/15 _x86_64_ (2 CPU)
15:49:28 CPU %user %nice %system %iowait %steal %idle
15:49:33 all 4.37 0.00 3.55 91.47 0.00 0.61
15:49:38 all 0.71 0.00 2.64 91.48 0.00 5.17
15:49:43 all 0.41 0.00 2.64 96.95 0.00 0.00
15:49:48 all 0.91 0.00 3.04 96.05 0.00 0.00
15:49:53 all 2.53 0.00 5.16 91.71 0.00 0.61
Average: all 1.78 0.00 3.41 93.53 0.00 1.28
So as you can see I am experiencing very high io wait
Now I only have 2 disks in my server i.e. /dev/sda and /dev/sdb
Q1. Why does Linux create dm-0,dm-1 and dm-2 as separate devices ( albeit I guess these are virtual devices via Device Manager ?
As far as I can tell these are the Oracle Linux Home, the Swap Device and
the Oracle Linux Root - however I don't see a command directly linking dm-0, and dm-1 with the /home and / mount points
Q2. How do you tell if the dm-0, dm-1 and dm-2 are using the sda or sdb device ?
Q3. I see dm-0 ( Linux Home ) is experiencing a high rate of tps ( transactions per second ? ) whereas the sda device ( which I believe dm-0 is ultimately on ) is experiencing a high amount of data read - is this where my performance problem resides ?
Q4. Is there a way to tell which mounted file system is performing poorly ? or any other way to determine why my io wait is so high ?
Q5. Why when I increase the Swap from 3 Gb to 19 Gb, I do this by adding a swap file ? Why is the 3 Gb shown as a swap device but the additional 16 Gb is not shown as a device ?
lab3:/root>swapon
NAME TYPE SIZE USED PRIO
/swapfile1 file 16G 6.8G -1
/dev/dm-1 partition 3G 0B -2
any help greatly appreciated
Jim