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Full Discussion: Disk Performance
Operating Systems Linux Disk Performance Post 302961567 by jimthompson on Tuesday 1st of December 2015 10:28:34 AM
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

Code:
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)

15:19:06        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 ?

Code:
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

Last edited by Scrutinizer; 12-01-2015 at 12:18 PM.. Reason: Code tags
 

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SWAPON(8)						     Linux Programmer's Manual							 SWAPON(8)

NAME
swapon, swapoff - enable/disable devices and files for paging and swapping SYNOPSIS
/sbin/swapon [-h -V] /sbin/swapon -a [-v] [-e] /sbin/swapon [-v] [-p priority] specialfile ... /sbin/swapon [-s] /sbin/swapoff [-h -V] /sbin/swapoff -a /sbin/swapoff specialfile ... DESCRIPTION
Swapon is used to specify devices on which paging and swapping are to take place. Calls to swapon normally occur in the system multi-user initialization file /etc/rc making all swap devices available, so that the paging and swapping activity is interleaved across several devices and files. Normally, the first form is used: -h Provide help -V Display version -s Display swap usage summary by device. Equivalent to "cat /proc/swaps". Not available before Linux 2.1.25. -a All devices marked as ``swap'' swap devices in /etc/fstab are made available. Devices that are already running as swap are silently skipped. -e When -a is used with swapon, -e makes swapon silently skip devices that do not exist. -p priority Specify priority for swapon. This option is only available if swapon was compiled under and is used under a 1.3.2 or later kernel. priority is a value between 0 and 32767. See swapon(2) for a full description of swap priorities. Add pri=value to the option field of /etc/fstab for use with swapon -a. Swapoff disables swapping on the specified devices and files. When the -a flag is given, swapping is disabled on all known swap devices and files (as found in /proc/swaps or /etc/fstab). NOTE
You should not use swapon on a file with holes. Swap over NFS may not work. SEE ALSO
swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(8) FILES
/dev/hd?? standard paging devices /dev/sd?? standard (SCSI) paging devices /etc/fstab ascii filesystem description table HISTORY
The swapon command appeared in 4.0BSD. Linux 1.x 25 September 1995 SWAPON(8)
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