Disk Performance

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Operating Systems Linux Disk Performance
# 1  
Old 12-01-2015
Disk Performance

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

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)

15:19:06        CPU     %user     %nice   %system   %iowait    %steal     %idle
15:19:11        all      0.50      0.00      0.40      3.52      0.00     95.58
15:19:16        all      0.50      0.00      0.50      2.21      0.00     96.78
15:19:21        all      0.70      0.00      0.40      1.81      0.00     97.08
15:19:26        all      0.40      0.00      0.40      3.73      0.00     95.46
15:19:31        all      0.50      0.00      0.50     13.29      0.00     85.70
Average:        all      0.52      0.00      0.44      4.91      0.00     94.12

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 Idon't see a command directly linking dm-0, and dm-1 which 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 ultamately 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 ?

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 ?

/swapfile1 file       16G 6.8G   -1
/dev/dm-1  partition   3G   0B   -2

any help greatly appreciated

Last edited by Scrutinizer; 12-01-2015 at 12:18 PM.. Reason: Code tags
# 2  
Old 12-09-2015
Wow ... This looks like a fun one Smilie

What is your environment? What did you install?

I need you to describe what your environment/application.
What software did you install and why?

That's for starters Smilie

# 3  
Old 12-09-2015
Check out the output of (output here is from one of my machines)..
ls -lart /dev/dm*
brw-rw----. 1 root disk253, 1 Nov 25 10:06 /dev/dm-1
brw-rw----. 1 root disk 253, 0 Nov 25 10:06 /dev/dm-0
brw-rw----. 1 root disk 253, 4 Nov 25 10:06 /dev/dm-4
brw-rw----. 1 root disk 253, 3 Nov 25 10:06 /dev/dm-3
brw-rw----. 1 root disk 253, 2 Nov 25 10:06 /dev/dm-2

Notice the MAJOR,MINOR numbers above
# dmsetup info
Name:              rootvg-tmp
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        1
Event number:      0
Major, minor:      253, 4
Number of targets: 1
..... # other output.

So rootvg-tmp coresponds with /dev/dm-4 device.

vgdisplay -v will provide output which physical volumes belong to which volume group.
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