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The Swap File SystemObjective: Explain virtual memory concepts and, given a scenario, configure and manage swap space. Physical memory is the random-access memory (RAM) installed in a computer. To view the amount of physical memory installed in your computer, type the following: prtconf| grep "Memory size" The system displays a message similar to the following: Memory size: 384 Megabytes Not all physical memory is available for Solaris processes. Some memory is reserved for kernel code and data structures. The remaining memory is referred to as available memory. Processes and applications on a system can use available memory. Physical memory is supplemented by specially configured space on the physical disk that is known as swap space; together they are referred to as virtual memory. Swap space is configured either on a special disk partition known as a swap partition or on a swap file system (swapfs). In addition to swap partitions, special files called swap files can also be configured in existing Unix file systems (UFS) to provide additional swap space when needed. Every process running on a Solaris system requires space in memory. Space is allocated to processes in units known as pages. Some of a process's pages are used to store the process executable, andand other pages are used to store the process's data. Physical memory is a finite resource on any computer, and sometimes there are not enough pages in physical memory for all of a system's processes. When a physical memory shortfall is encountered, the virtual memory system begins moving data from physical memory out to the system's configured swap areas. When a process requests data that has been sent to a swap area, the virtual memory system brings that data back into physical memory. This process is known as paging. The Solaris virtual memory system maps the files on disk to virtual addresses in memorythis is referred to as virtual swap space. As data in those files is needed, the virtual memory system maps the virtual addresses in memory to real physical addresses in memory. This mapping process greatly reduces the need for large amounts of physical swap space on systems with large amounts of available memory. The virtual swap space provided by swapfs reduces the need for configuring large amounts of disk-based swap space on systems with large amounts of physical memory. This is because swapfs provides virtual swap space addresses rather than real physical swap space addresses in response to the requests to reserve swap space. With the virtual swap space provided by swapfs, real disk-based swap space is required only with the onset of paging, because when paging occurs, processes are contending for memory. In this situation, swapfs must convert the virtual swap space addresses to physical swap space addresses in order for paging to actual disk-based swap space to occur. Swap Space and TMPFSThe temporary file system (TMPFS) makes use of virtual memory for its storagethis can be either physical RAM or swap space; it is transparent to the user. /tmp is a good example of a TMPFS file system where temporary files and their associated information are stored in memory (in the /tmp directory) rather than on disk. This speeds up access to those files and results in a major performance enhancement for applications such as compilers and database management system (DBMS) products that use /tmp heavily. TMPFS allocates space in the /tmp directory from the system's virtual memory resources. This means that as you use up space in /tmp, you are also using up virtual memory space. So if your applications use /tmp heavily and you do not monitor virtual memory usage, your system could run out of this resource. Sizing Swap SpaceThe amount of swap space required on a system is based on the following criteria:
Note Movement of Swap Starting with the release of Solaris 9, the installation program allocates swap at the first available cylinder on the disk (this is normally cylinder 0). This practice allows the root file system the maximum space on the disk and allows for expansion of the file system during an upgrade. The amount of disk-based swap space on a system must be large enough to be able to accommodate a kernel memory dump, plus the requirements of any concurrently running processes, including third-party applications and compilers. Many other factors also contribute to the amount of swap space you need to configure, such as the number of concurrent users and the naming service, Network Information System Plus (NIS+). It is quite rare nowadays to need more swap space than RAM, which used to be a recommendation with older versions of SunOS; in fact the opposite is often trueyou now often need less swap space than physical RAM. If you are prepared to keep track of your swap space and administer it regularly, you can run with much less swap space than in older versions of SunOS. (How to monitor swap space and how to add additional space to a running system are discussed in the next few sections.) Note Reducing Swap Space Problems If the amount of swap space is equal to the amount of physical RAM, you should generally experience no swap space problems, although the type of application being used on the system will be a major factor. Monitoring Swap ResourcesIf you run into a swap shortfall due to heavy demand on memory, you get error messages on your system's console. The error might look something like this: <application> is out of memory
malloc error O
messages.1:SJul 18 15:12:47 ultra genunix: [ID 470503 kern.warning]
WARNING: Sorry, no swap space to grow stack for pid 100295 (myprog)This error means that an application is trying to get more memory and there is no swap space available to back it. You could fill up a TMPFS due to the lack of available swap and get the following error message: <directory>: File system full, swap space limit exceededor this one: <directory>: File system full, memory allocation failedThis type of message is displayed if a page cannot be allocated when a file is being written. This can occur, for example, when TMPFS tries to write more than it is allowed or when TMPFS runs out of physical memory while attempting to create a new file or directory. You need to regularly monitor your swap space. This helps you determine whether you are running on the edge and need to increase the resource or maybe you have too much swap space allocated and are wasting disk space. Most commercial performance monitoring tools keep track of swap space or can be configured to generate warnings when it gets low. Besides these commercial tools, you can use the helpful tools that Solaris provides (see Table 9.1). System performance monitoring is not covered on the administrator certification exams, so this chapter describes only the /usr/sbin/swap command.
You can use two options with the /usr/sbin/swap command to monitor swap space. You can use the -l option to list swap space and to determine the location of a system's swap areas: swap -l The system displays details of the system's physical swap space. This system has a 512MB swap slice allocated, as shown below: swapfile dev swaplo blocks free /dev/dsk/c0t0d0s1 136,9 16 1049312 1049312 This output is described in Table 9.2.
You use the -s option to list a summary of the system's virtual swap space: swap -s The system displays the following information, which shows the details of the system's physical swap space and includes physical memory too. This system has 384MB of physical memory and a 512MB swap slice: total: 67648k bytes allocated + 19032k reserved = 86680k used, 724512k \ available This output is described in Table 9.3
You can use the amounts of swap space available and used (in the swap -s output) as a way to monitor swap space usage over time. If a system's performance is good, you can use swap -s to see how much swap space is available. When the performance of a system slows down, you can check the amount of swap space available to see if it has decreased. Then you can identify what changes to the system might have caused swap space usage to increase. Keep in mind when using the swap command that the amount of physical memory available for swap usage changes dynamically as the kernel and user processes reserve and release physical memory. Note Swap Space Calculations The swap -l command displays swap space in 512-byte blocks, and the swap -s command displays swap space in 1,024-byte blocks. If you add up the blocks from swap -l and convert them to kilobytes, you'll see that it is less than the swap space used plus available (as shown in the swap -s output) because swap -l does not include physical memory in its calculation of swap space. Setting Up Swap SpaceSwap space is initially configured during software installation through the installation program. If you use the installation program's automatic layout of disk slices and do not manually change the size of the swap slice, the Solaris installation program allocates a default swap slice of 512MB. Note Crash Dumps As described later in this chapter, a crash dump is a disk copy of the kernel memory of the computer at the time of a fatal system error. When a fatal operating system error occurs, a message describing the error is printed to the console. The operating system then generates a crash dump by writing the contents of kernel memory to a predetermined dump device, which is typically a local disk partition. You can then analyze this crash dump to determine the cause of the system error. By default, the dump device is configured to be an appropriate swap partition. Therefore, it's necessary to make sure that your swap area is at least as large as about 25% of your physical RAM; otherwise, the system may not have enough room to store the crash dump. Crash dumps and core files are discussed later in this chapter, in the sections "Core File Configuration" and "Crash Dump Configuration." The software installation program adds entries for swap slices and files in the /etc/vfstab file. These swap areas are activated each time the system is booted by /sbin/swapadd. As system configurations change, more users are added, and new software packages are installed, you might need to add more swap space. There are two methods for adding more swap to a system:
Creating a secondary swap partition requires additional, unused disk space. You use the format command as described in Chapter 1, "Managing File Systems," to create a new partition and file system on a disk. After you create the swap partition, you make an entry in the /etc/vfstab file so that the swap space is activated at bootup. The process is described in Step by Step 9.1.
Exam Alert /etc/vfstab syntax You should be familiar with the entry for swap files in /etc/vfstab. The syntax can be tricky, especially because of the hyphens. The following are additional notes that explain how to add swap partitions:
The easiest way to add more swap space is to use the mkfile and swap commands to designate a part of an existing UFS as a supplementary swap area. You can do this as a temporary or semi-temporary solution for a swap shortage. Although you can do this for longer durations as well, it has a few disadvantages:
Step by Step 9.2 explains how to add more swap space without repartitioning a disk.
There is some disagreement as to which type of swap area provides the best performance: a swap partition or a swap file. There are factors in favor of both scenarios; however, these are two of the best reasons in favor of swap partitions:
Sun's official statement, and the general consensus in the user community, is that there will be a performance impact if you go the swap file route rather than the partition route. Sun recommends that you use swap files only as a temporary solution, until you can add a swap partition. Note Swap Files on NFS In an emergency, when no other local space is available, it's possible to add a swap file to a networked file system by using NFS; this is described later in this chapter. Using NFS to access swap space on another host is not recommended, however, because it puts an increased load on your network and makes performance unacceptable. If you do need to use NFS for additional swap files, try using the -n option when you run mkfile, as this will only allocate disk blocks as they are written. Swap files can be deleted as well as added. For example, you might determine that you have allocated too much swap space and that you need that disk space for other uses. Alternatively the additional swap space might have been temporarily added to accommodate a one-off large job. The steps involved in removing a swap file are outlined in Step by Step 9.3.  |
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