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Swap Space Management

Sun, 03/05/2000 - 17:01 — charlie.collins

Managing the swap properly can make a big difference, either way, to system performance. This handy mini swap space tutorial is an excellent reference. Thanks to the Outrider Computing Journal for the story.


Swap This! Swap Space Management

Not too long ago, OutRider had a column about memory. This column will look at swap space allocation for small workstations to low end enterprise size systems (ranging anywhere from 32MB to 50GB). This is because I do not
have experience beyond those ranges and when dealing with architectures beyond that range, you are looking at network wide load balancing etc., all of which is beyond the scope of this document.

Having a Reference Point

Having a solid reference point is perhaps the best possible situation an administrator can find themselves in. An instance when this might occur might be if one is transferring a particular set(s) of software and function(s) from an
older architecture to a newer one. In that particular instance, the manufacturers [ 1 ], user groups and vendors all will usually have sets of comparisons about the differences between one system versus another and (if you are lucky)
information about particular application [ 2 ] needs where swap space is concerned.

Another way to beat the problem to the punch is to attempt to test out a particular set of applications on a system before completing the configuration of a newer one. This is a much more difficult situation to be in, but I have
actually been there as it were, it worked out for the best.

One thing I have consistently discovered is when I do have the occasion to test an application prior to deployment, or I already have a reference point, every single time (so far that is) I have grossly overestimated my needs. I believe
this is because I do not fully take into account faster bus speeds etc. and a small degree of incredible luck. I can only hope all of my fellow administrators are as haplessly fortunate as I have been, however, by the same token, I have
suddenly the extra resources swallowed up by unforeseen projects or events in general. In the end it all seems to balance out.

Determining Required Swap Space

There are several methods for determining the amount of swap space you need. This column will examine four methods:

  • The standard swap space rule
  • Following Recommended Guidelines
  • Sizing from a Reference Point
  • Combining methods & knowing the system

    Of course, the last one is not a stand alone method, but it deserves special attention.

    The Standard Swap Space Rule

    In a nutshell, (RAM x 2). For instance, if you had 24GB of memory, 48GB of swap space. While this practice generally works, it is not always the most efficient. Some flavors of UNIX take special advantage of swap space, while others may not. A system may not even need any (although that is definitely a rarity - it is always a good idea
    to have some).

    Following Recommended Guidelines

    This particular method can be helpful, but it requires a great deal of research. To coin a phrase from a popular movie:

    \"The sheer logistics of it are mind boggling\"

    Which can certainly be true of determining swap space needs based on:

  • Getting opinions from scsi and disk vendors
  • Getting opinions from memory vendors
  • Getting opinions from anyone who sells hardware
    . . .

    I think you get the picture.

    All is not lost, however. Upon further investigation, one can find all sorts of information to help them make a well
    informed decision about swap space requirements. Here are a couple of suggestions:

  • Newsgroups
  • User Groups
  • Trade Journals
  • Peers and friends who may be in the same field

    Sizing From A Reference Point

    By far the easiest method is to size from a given reference point. The applied method of doing this is actually somewhat tricky, however, you will find this method in a great deal of trade journals and books (actually I have seen this in several books).

    There is a deviation of this method that works on systems that are not yet online (i.e. the system you are about put online). Simply setup a grossly oversized temporary swapfile to hold you over until you create the swap partition(s).

    Basically you have a bunch of knowns, these knowns are how much memory your system will require when running x applications with n instances, following is the exact method:

  • 1.Estimate your total memory needs by adding up the amount of memory used by as many running programs you believe the system might run at once and the instances of them, for example, if 6 users constantly run a calender program, you could run it then multiply the amount by 6 (taking into account standard login and shell memory usage as well).
  • 2.Add some bufferspace to your estimates based upong the amount of RAM you have, for example, if you have 64MB of RAM, a couple of megabytes will do, if you have 50 Gigabytes of RAM, a couple fo Gigabytes should suffice.
  • 3.Subtract the amount of physical memory from this total. The remainder is the amount of swap you will need.
  • 4.If the amount in step 3 is three times the size of physical memory, you might run into some problems. If the results show that you do not need any swap space, make some anyway in case your system uses it on a convienence basis.

    This method, however, is not fool proof. There are instances when a certain architecture will simply require what might seem like more than you will ever use.

    Combining Methods and Knowing Your System

    As I mentioned earlier, no method is full proof (or fool proof as it were). There are instances when even the old Ram x 2 will not suffice. The reason for this is some applications (databases in particular) are designed to make use of swap in some sort of special way. That being the case, the normal rules may not apply. Additionally, there is always the unforeseen additions and loads that come into play.

    Finally, having an initmate knowledge of your systems helps. As an example, had I known the precise bus architecture on a new ssystem, I might have saved some money by not getting the amount of RAM I did initially.

    We have covered the methods of determining swap space requirements, next we will examine the definitions of swapfiles and partitions, the pros and cons of each and how to set them up.

    Swap Partitions and Swap Files

    A swap partition is a predefined and allocated disk area that is used only for swapping. A swapfile is, as it intuitively
    sounds, a file that does the same job, however, it can grow to the size of the partition it resides on. With that in mind,
    creating a swapfile on say a /usr partition may not be the wisest idea.

    Swapfiles are best employed as a temporary holdover measure, or as I mentioned above, a temporary device for
    determining swap partition needs. A swapfile may also work well on a small workstation, otherwise, the keyword to
    remember here is temporary.

    Activating and Shutting Off Swapfiles

    The syntax for activating a swapfile is pretty simple:

    swapon

    For example, if we made a temporary swapfile called /var/swapthis:

    swapon /var/swapthis

    Allocating a swap partition varies from UNIX system to system, however, the most common command to create swap
    space is the mkswap command. In the following example, I am creating a swap partition on a logical volume in
    volume group 00:

    mkswap /dev/vg00/lvol04

    Activating swap partitions is similar to files, using the logical volume example, the syntax would look like so:

    swapon /dev/vg00/lvol04

    Turning off swap uses the swapoff command. Using the swapfile example, the command would look like so:

    swapoff /var/swapthis

    As you can see, creating and undoing swap space is relatively simple [ 4 ].

    Swap Space Priority

    Swap space performance can be enhanced (or worsened) by altering priorities. On some systems, there may be
    multiple swap partitions on different devices. Since priority is based upon \"first come first serve\" by default. So, you
    may have an instance where you would want to alter default priorities because of a new disk installation etc. The
    syntax usually something like this:

    swapon -p

    With the priority being a number. In most cases, the higher the number, the higher the priority.

    Last Comments on Swap Space Performance

    Swap partitions are disks, keeping that in mind, many principles that one might apply to disks can also be apoplied to
    swap space. Using RAID or striping in general can increase performance. Try to allocate swap partitions near the
    beginning of a drive etc.

    As Usual, This is Not the Endall Document

    Any performance issue can never be fully examined in a short column, as I stated earlier, this applies to small to low
    end enterprise systems. Even so, there will be instances where this particular column may have missed the mark. I
    highly recommend visiting newsgroups, consulting local documentation and manuals to learn more about your
    particular UNIX system interacts with swap space.

    Yes, its not all science in this document, it lacks some refinement, but it is a good starting point.   Outrider Computing Journal