Optimizing AIX 5L performance: Tuning disk performance, Part 2

Monitoring logical volumes and analyzing the results

Discover how to use appropriate disk placement prior to creating your logical volumes to improve disk performance. Part 2 of this series (see Resources) focuses on monitoring your logical volumes and the commands and utilities (iostat, lvmstat, lslv, lspv, and lsvg) used to analyze results.

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Ken Milberg, Future Tech UNIX Consultant, Technology Writer, and Site Expert, Future Tech

Ken Milberg is a Technology Writer and Site Expert for techtarget.com and provides Linux technical information and support at searchopensource.com. He is also a writer and technical editor for IBM Systems Magazine, Open Edition. Ken holds a bachelor's degree in computer and information science and a master's degree in technology management from the University of Maryland. He is the founder and group leader of the NY Metro POWER-AIX/Linux Users Group. Through the years, he has worked for both large and small organizations and has held diverse positions from CIO to Senior AIX Engineer. Today, he works for Future Tech, a Long Island-based IBM business partner. Ken is a PMI certified Project Management Professional (PMP), an IBM Certified Advanced Technical Expert (CATE, IBM System p5 2006), and a Solaris Certified Network Administrator (SCNA). You can contact him at kmilberg@gmail.com.



24 July 2007

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About this series

This three-part series (see Resources) on the AIX® disk and I/O subsystem focuses on the challenges of optimizing disk I/O performance. While disk tuning is arguably less exciting than CPU or memory tuning, it is a crucial component in optimizing server performance. In fact, partly because disk I/O is your weakest subsystem link, there is more you can do to improve disk I/O performance than any other subsystem.

Introduction

Unlike the tuning of other subsystems, tuning disk I/O should actually be started during the architectural phase of building your systems. While there are virtual memory equivalents of I/O tuning parameters (ioo and lvmo), the best way to increase disk I/O performance is by properly configuring your systems and not tuning parameters. Unlike virtual memory tuning, it is much more complex to change the way in which you structure your logical volumes after they have been created and running, so you usually get only one chance to do this right. This article discusses the ways that you can configure your logical volumes and where to actually place them with respect to the physical disk, and it also addresses the tools used to monitor your logical volumes. Most of these tools are not meant to be used for long-term trending and are specific AIX tools that provide information as to how the logical volumes are configured and if they have been optimized for your environment.

Part 1 (see Resources) of this series introduced iostat, but it did not address using the tool outside of viewing Asynchronous I/O servers. Part 2 uses iostat to monitor your disks and shows you what it can do to help quickly determine your I/O bottleneck. While iostat is one of those generic UNIX® utilities that was not developed specifically for AIX, it is actually very useful for quickly determining what is going on in your system. The more specific AIX logical volume commands help drill down deeper into your logical volumes to help you really analyze what your problems are, if any. It's important that you clearly understand what you're looking for before using these tools. This article describes the tools and also shows you how to analyze their output, which helps in analyzing your disk I/O subsystem.

Logical volume and disk placement overview

This section defines the Logical Volume Manager (LVM) and introduces some of its features. Let's drill down into logical volume concepts, examine how they relate to improving disk I/O utilization, and talk about logical volume placement as it relates to the physical disk, by defining and discussing both intra-policy and inter-policy disk practices.

Conceptually, the logical volume layer sits between the application and physical layers. In the context of disk I/O, the application layers are the file system or raw logical volumes. The physical layer consists of the actual disk. LVM is an AIX disk management system that maps the data between logical and physical storage. This allows data to reside on multiple physical platters and to be managed and analyzed using specialized LVM commands. LVM actually controls all the physical disk resources on your system and helps provide a logical view of your storage subsystem. Understanding that it sits between the application layer and the physical layer should help you understand why it is arguably the most important of all the layers. Even your physical volumes themselves are part of the logical layer, as the physical layer only encompasses the actual disks, device drivers, and any arrays that you might have already configured. Figure 1 illustrates the concepts and shows how tightly integrated the logical I/O components relate to the physical disk and its application layer.

Figure 1. Logical volume diagram
Logical volume diagram

Let's now quickly introduce the elements that are part of LVM, from the bottom up. Each of the drives is named as a physical volume. Multiple physical volumes make up a volume group. Within the volume groups, logical volumes are defined. The LVM enables the data to be on multiple physical drives, though they might be configured to be on a single volume group. These logical volumes can be either one or multiple logical partitions. Each of the logical partitions has a physical partition that correlates to it. Here is where you can have multiple copies of the physical portions for purposes such as disk mirroring.

Let's take a quick look at how logical volume creation correlates with physical volumes. Figure 2 illustrates the actual storage position on the physical disk platter.

Figure 2. Actual storage position on the physical disk platter
Actual storage position on the physical disk platter

As a general rule, data that is written toward its center has faster seek times than data written on the outer edge. This has to do with the density of data. Because it is more dense as it moves toward its center, there is actually less movement of the head. The inner edge usually has the slowest seek times. As a best practice, the more intensive I/O applications should be brought closer to the center of the physical volumes. Note that there are exceptions to this. Disks hold more data per track on the edges of the disk, not on the center. That being said, logical volumes being accessed sequentially should actually be placed on the edge for better performance. The same holds true for logical volumes that have Mirror Write Consistency Check (MWCC) turned on. This is because the MWCC sector is on the edge of the disk and not at the center of it, which relates to the intra-disk policy of logical volumes.

Let's discuss another important concept referred to as the inter-disk policy of logical volumes. The inter-disk policy defines the number of disks on which the physical partitions of a logical volume actually resides. The general rule is that the minimum policy provides the greatest reliably and availability, and the maximum policy improves performance. Simply put, the more drives that data is spread on, the better the performance. Some other best practices include: allocating intensive logical volumes to separate physical volumes, defining the logical volumes to the maximum size you need, and placing logical volumes that are frequently used close together. This is why it is so important to know your data prior to configuring your systems so that you can create policies that make sense from the start.

You can define your polices when creating the logical volumes themselves using the following command or smit fastpath: # mklv or # smitty mklv.

Monitoring logical volumes and analyzing results

This section provides instructions on how to monitor your logical volumes and analyze the results. Various commands are introduced along with the purposes for which they are used, and you examine the output as well.

A ticket has just been opened up with the service desk that relates to slow performance on some database server. You suspect that there might be an I/O issue, so you start with iostat. If you recall, this command was introduced in the first installment of the series (see Resources), though only for the purposes of viewing asynchronous I/O servers. Now, let's look at iostat in more detail. iostat, the equivalent of using vmstat for virtual memory, is arguably the most effective way to get a first glance of what is happening with your I/O subsystem.

Listing 1. Using iostat
# iostat 1

System configuration: lcpu=4 disk=4

tty:      tin         tout   avg-cpu:  % user    % sys     % idle    % iowait
          0.0        392.0               5.2      5.5       88.3       1.1

Disks:        % tm_act     Kbps      tps    Kb_read   Kb_wrtn
hdisk1           0.5      19.5       1.4   53437739  21482563
hdisk0           0.7      29.7       3.0   93086751  21482563
hdisk4           1.7     278.2       6.2   238584732  832883320
hdisk3           2.1     294.3       8.0   300653060  832883320

What are you seeing here and what does this all mean?

  • % tm_act: Reports back the percentage of time that the physical disk was active or the total time of disk requests.
  • Kbps: Reports back the amount of data transferred to the drive in kilobytes.
  • tps: Reports back the number of transfers per second issued to the physical disk.
  • Kb_read: Reports back the total data (kilobytes) from your measured interval that is read from the physical volumes.
  • Kb_wrtn: Reports back the amount of data (kilobytes) from your measured interval that is written to the physical volumes.

You need to watch % tm_act very carefully, because when its utilization exceeds roughly 60 to 70 percent, it usually is indicative that processes are starting to wait for I/O. This might be your first clue of impending I/O problems. Moving data to less busy drives can obviously help ease this burden. Generally speaking, the more drives that your data hits, the better. Just like anything else, too much of a good thing can also be bad, as you have to make sure you don't have too many drives hitting any one adapter. One way to determine if an adapter is saturated is to sum the Kbps amounts for all disks attached to one adapter. The total should be below the disk adapter throughput rating, usually less than 70 percent.

Using the -a flag (see Listing 2) helps you drill down further to examine adapter utilization.

Listing 2. Using iostat with the -a flag
# iostat -a

Adapter:                   Kbps      tps    Kb_read   Kb_wrtn
scsi0                      0.0       0.0          0         0

Paths/Disk:       % tm_act     Kbps      tps    	Kb_read   Kb_wrtn
hdisk1_Path0           37.0       89.0     0.0          0         0
hdisk0_Path0           67.0       47.0     0.0          0         0
hdisk4_Path0           0.0        0.0      0.0          0         0
hdisk3_Path0           0.0        0.0      0.0          0         0

Adapter:                   Kbps      tps    Kb_read   Kb_wrtn
ide0                       0.0       0.0          0         0

Paths/Disk:       % tm_act     Kbps      tps    Kb_read   Kb_wrtn
cd0                    0.0       0.0       0.0          0         0

Clearly, there are no bottlenecks here. Using the -d flag allows you to drill down to one specific disk (see Listing 3).

Listing 3. Using iostat with the -d flag
# iostat -d hdisk1 1

System configuration: lcpu=4 disk=5

Disks:        % tm_act     Kbps      tps    Kb_read   Kb_wrtn
hdisk1           0.5      19.4       1.4   53437743  21490480
hdisk1           5.0      78.0       23.6         3633      3564
hdisk1           0.0       0.0       0.0          0         0
hdisk1           0.0       0.0       0.0          0         0
hdisk1           0.0       0.0       0.0          0         0
hdisk1           0.0       0.0       0.0          0         0

Let's look at some specific AIX LVM commands. You examined disk placement earlier and the importance of architecting your systems correctly from the beginning. Unfortunately, you don't always have that option. As system administrators, you sometimes inherit systems that must be fixed. Let's look at the layout of the logical volumes on disks to determine if you need to change definitions or re-arrange your data.

Let's look first at a volume group and find the logical volumes that are a part of it. lsvg is the command that provides volume group information (see Listing 4).

Listing 4. Using lsvg
# lsvg -l data2vg

Data2vg:
LV NAME             TYPE       LPs   PPs   PVs  LV STATE      MOUNT POINT
data2lv             jfs        128   256   2    open/syncd    /data2
loglv00             jfslog     1     2     2    open/syncd    N/A
appdatalv       	 jfs        128   256   2    open/syncd    /appdata

Now, let's use lslv, which provides for specific data on logical volumes (see Listing 5).

Listing 5. Using lslv
# lslv data2lv

LOGICAL VOLUME:     data2lv                VOLUME GROUP:   data2vg
LV IDENTIFIER:      0003a0ec00004c00000000fb076f3f41.1 PERMISSION:     read/write
VG STATE:           active/complete        LV STATE:       opened/syncd
TYPE:               jfs                    WRITE VERIFY:   off
MAX LPs:            512                    PP SIZE:        64 megabyte(s)
COPIES:             2                      SCHED POLICY:   parallel
LPs:                128                    PPs:            256
STALE PPs:          0                      BB POLICY:      relocatable
INTER-POLICY:       minimum                RELOCATABLE:    yes
INTRA-POLICY:       center                 UPPER BOUND:    32
MOUNT POINT:        /data                  LABEL:          /data
MIRROR WRITE CONSISTENCY: on/ACTIVE
EACH LP COPY ON A SEPARATE PV ?: yes
Serialize IO ?:     NO

This view provides a detailed description of your logical volume attributes. What do you have here? The intra-policy is at the center, which is normally the best policy to have for I/O-intensive logical volumes. As you recall from an earlier discussion, there are exceptions to this rule. Unfortunately, you've just hit one of them. Because Mirror Write Consistency (MWC) is on, the volume would have been better served if it were placed on the edge. Let's look at its inter-policy. The inter-policy is minimum, which is usually the best policy to have if availability is more important then performance. Further, there are double the number of physical partitions than logical partitions, which signify that you are mirroring your systems. In this case, you were told that raw performance was the most important objective, so the logical volume was not configured in such a way as to the reality of how the volume is being utilized. Further, if you are mirroring your system and using an external storage array, this would even be worse, as you're already providing mirroring at the hardware layer, which is actually more effective then using AIX mirroring.

Let's drill down even further in Listing 6.

Listing 6. lslv with the -l flag
# lslv -l data2lv

data2lv:/data2
PV                COPIES        IN BAND       DISTRIBUTION
hdisk2            128:000:000   100%          000:108:020:000:000
hdisk3            128:000:000   100%          000:108:020:000:000

The -l flag of lslv lists all the physical volumes associated with the logical volumes and distribution for each logical volume. You can then determine that 100 percent of the physical partitions on the disk are allocated to this logical volume. The distribution sections show the actual number of physical partitions within each physical volume. From here, you can detail its intra-disk policy. The order of these fields are as follows:

  • Edge
  • Middle
  • Center
  • Inner-middle
  • Inner-edge

The reports show that most of the data is in the middle and some at the center.

Let's keep going and find out which logical volumes are associated with the one physical volume. This is done with the lspv command (see Listing 7).

Listing 7. Using the lspv command
# lspv -l hdisk2

hdisk2:
LV NAME               LPs   PPs     DISTRIBUTION          MOUNT POINT
loglv01               1     1       01..00..00..00..00    N/A
data2lv               128   128     00..108..20..00..00   /data2
appdatalv    	      128   128     00..00..88..40..00    /appdata

Now you can actually identify which of the logical volumes on this disk are geared up for maximum performance.

You can drill down even further to get more specific (see Listing 8).

Listing 8. lspv with the -p flag
# lspv -p hdisk2

hdisk2:

PP RANGE  STATE   REGION        LV ID               TYPE       MOUNT POINT
  1-108   free    outer edge
109-109   used    outer edge    loglv00             jfslog     N/A
110-217   used    outer middle  data2lv             jfs         /data2
218-237   used    center        appdatalv             jfs        /appdata
238-325   used    center        testdatalv          jfs          /testdata
326-365   used    inner middle  stagingdatalv        jfs        /staging
366-433   free    inner middle
434-542   free    inner edge

This view tells you what is free on the physical volume, what has been used, and which partitions are used where. This is a nice view.

One of the best tools to look at LVM usage is with lvmstat (see Listing 9).

Listing 9. Using lvmstat
# lvmstat -v data2vg
0516-1309 lvmstat: Statistics collection is not enabled for this logical device.
        Use -e option to enable.

As you can see by the output here, it is not enabled (by default), so you need to actually enable it prior to running the tool using # lvmstat -v data2vg -e. The following command takes a snapshot of LVM information every second for 10 intervals:

# lvmstat -v data2vg 1 10

This view shows the most utilized logical volumes on your system since you started the data collection tool. This is very helpful when drilling down to the logical volume layer when tuning your systems (see Listing 10).

Listing 10. lvmstat with the -v flag
# lvmstat -v data2vg

Logical Volume       iocnt     Kb_read              Kb_wrtn      Kbps
  appdatalv           306653    47493022            383822      103.2
  loglv00                 34        0                3340       2.8
  data2lv                453      234543            234343       89.3

What are you looking at here?

  • % iocnt: Reports back the number of read and write requests.
  • Kb_read: Reports back the total data (kilobytes) from your measured interval that is read.
  • Kb_wrtn: Reports back the amount of data (kilobytes) from your measured interval that is written.
  • Kbps: Reports back the amount of data transferred in kilobytes.

Look at the man pages for all the commands discussed before you start to add them to your repertoire.

Tuning with lvmo

This section goes over using a specific logical volume tuning command. The lvmo is used to set and display your pbuf tuning parameters. It is also used to display blocked I/O statistics.

lvmo is one of those new commands first introduced in AIX Version 5.3. It's important to note that the usage of the lvmo command allows changes for LVM pbuf tunables only that are dedicated to specific volume groups. The ioo utility is still the only way to manage pbufs on a system-wide basis. This is because prior to AIX Version 5.3, the pbuf pool parameter was a system-wide resource. With the introduction of AIX Version 5.3, LVM manages one pbuf pool for each volume group. What is a pbuf? A pbuf is best defined as a pinned memory buffer. LVM uses these pbufs to control pending disk I/O operations.

Let's display your lvmo tunables for the data2vg volume group (see Listing 11).

Listing 11. Displaying lvmo tunables
# lvmo -v data2vg -a

vgname = data2vg
pv_pbuf_count  = 1024
total_vg_pbubs = 1024
mag_vg_pbuf_count = 8192
perv_blocked_io_count = 7455
global_pbuf_count = 1024
global_blocked_io_count = 7455

What are the tunables here?

  • pv_pbuf_count: Reports back the number of pbufs added when a physical volume is added to the volume group.
  • Max_vg_pbuf_count: Reports back the max amount of pbufs that can be allocated for a volume group.
  • Global_pbuf_count: Reports back the number of pbufs that are added when a physical volume is added to any volume group.

Let's increase the pbuf count for this volume group:

# lvmo -v redvg -o pv_pbuf_count=2048

Quite honestly, I usually stay away from lvmo and use ioo. I'm more used to tuning the global parameters. It's important to note that if you increase the pbuf value too much, you can actually see a degradation in performance.

Conclusion

This article focused on logical volumes and how they relate to the disk I/O subsystem. It defined logical volumes at a high level and illustrated how it relates to the application and physical layers. It also defined and discussed some best practices for inter-disk and intra-disk polices as they relate to creating and maintaining logical volumes. You looked at ways to monitor I/O usage for your logical volumes, and you analyzed the data that was captured from the commands that were used to help determine what your problems were. Finally, you actually tuned your logical volumes by determining and increasing the amount of pbufs used in a specific volume group. Part 3 of this series focuses on the application layer as you move on to file systems, using various commands to monitor and tune your file systems and disk I/O subsystems.

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