Adding and Managing Fedora Swap Space
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An important part of maintaining the performance of a Fedora system involves ensuring that adequate swap space is available relative to the memory demands placed on the system. The goal of this chapter, therefore, is to provide an overview of swap management on Fedora.
What is Swap Space?
Computer systems have a finite amount of physical memory that is made available to the operating system. When the operating system begins to approach the limit of the available memory it frees up space by writing memory pages to disk. When any of those pages are required by the operating system they are subsequently read back into memory. The area of the disk allocated for this task is referred to as swap space.
Recommended Swap Space for Fedora
The amount of swap recommended for Fedora depends on a number of factors including the amount of memory in the system and the workload imposed on that memory. The current guidelines for Fedora swap space are as follows:
- 4GB of RAM requires a minimum of 2GB of swap space
- 4GB to 16GB RAM requires a minimum of 4GB of swap space
- 16GB to 64GB of RAM requires a minimum of 8GB of swap space
- 64GB to 256GB of RAM requires a minimum of 16GB of swap space
Identifying Current Swap Space Usage
The current amount of swap used by a Fedora system may be identified in a number of ways. One option is to cat the /proc/swaps file:
# cat /proc/swaps Filename Type Size Used Priority /dev/dm-1 partition 2031608 0 -1
Alternatively, use the swapon command:
# swapon -s Filename Type Size Used Priority /dev/dm-1 partition 2031608 0 -1
Finally, the free command may also be used:
# free
total used free shared buffers cached
Mem: 1027220 969300 57920 0 57072 665576
-/+ buffers/cache: 246652 780568
Swap: 2031608 0 2031608
Adding a Swap File to a Fedora System
Additional swap may be added to the system by creating a file and assigning it as swap. This is achieved as follows. Create the swap file using the dd command (the size can be changed by adjusting the count= variable; the following creates a 131MB file):
# dd if=/dev/zero of=/newswap bs=1024 count=128000 128000+0 records in 128000+0 records out 131072000 bytes (131 MB) copied, 1.7639 seconds, 74.3 MB/s
Configure the file as swap:
# mkswap /newswap Setting up swapspace version 1, size = 131067 kB
Add the swap file to the system in real-time:
# swapon /newswap
Finally, modify the /etc/fstab file to automatically add the new swap at system boot time by adding the following line:
/newswap swap swap defaults 0 0
Once the swap space has been activated, verify that it is in use using the swapon –s command:
# swapon -s Filename Type Size Used Priority /dev/dm-1 partition 2031608 0 -1 /newswap file 127992 0 -2
De-activate the additional swap space at any time using the swapoff command as follows:
# swapoff /newswap
Adding Swap to a Fedora LVM Swap Volume
By default, Fedora configures swap space using Logical Volume Management (LVM). An alternative to adding swap via file, therefore, is to extend the logical volume used for the swap.
The first step is to identify the current amount of swap available and the volume group and logical volume used for that swap space (for more information on LVM and how to add additional space, refer to the chapter entitled Adding a New Disk to a Fedora Volume Group and Logical Volume):
# lvdisplay --- Logical volume --- LV Name /dev/vg_fed2/lv_root VG Name vg_fed2 LV UUID XZ1Da9-1OdZ-em3Z-4Dfo-TqZl-V6WI-an3Ahq LV Write Access read/write LV Status available # open 1 LV Size 13.56 GiB Current LE 434 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 253:0 --- Logical volume --- LV Name /dev/vg_fed2/lv_swap VG Name vg_fed2 LV UUID 94ltyQ-aGe2-271U-tiZP-Xcuo-x8wm-E0rY5Z LV Write Access read/write LV Status available # open 1 LV Size 1.94 GiB Current LE 62 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 253:1
Clearly the swap resides on logical volume /dev/vg_fed2/lv_swap which is part of volume group vg_fed2. The next step is to verify if there is any space available on the volume group that can be allocated to the swap volume:
# vgs VG #PV #LV #SN Attr VSize VFree vg_fed2 2 2 0 wz--n- 63.84G 992.00M
If the amount of space available is sufficient to meet additional swap requirements, turn off the swap and extend the volume group to use the additional space:
# swapoff /dev/vg_fed2/lv_swap # lvextend -L +900M /dev/vg_fed2/lv_swap Rounding up size to full physical extent 928.00 MB Extending logical volume LogVol01 to 2.88 GB Logical volume LogVol01 successfully resized
Next, reformat the swap volume and turn the swap back on:
# mkswap /dev/vg_fed2/lv_swap Setting up swapspace version 1, size = 3087003 kB # swapon /dev/vg_fed2/lv_swap
Having made the changes check that the swap space has increased using the swapon –s command.
Adding Swap Space to the Volume Group
In the above section we extended the swap logical volume to use space that was already available in the volume group. If no space is available in the volume group then it will need to be added before the swap can be extended. Begin by checking the status of the volume group:
# vgs VG #PV #LV #SN Attr VSize VFree vg_fed2 1 2 0 wz--n- 15.50g 0
The above output indicates that no space is available within the volume group. Suppose, however, that we have a requirement to add 2GB to the swap on the system. Clearly, this will require the addition of more space to the volume group. For the purposes of this example it will be assumed that a disk partition that is 2GB is size and represented by /dev/sdb is available for addition to the volume group. The first step is to turn this partition into a physical volume:
# pvcreate –ff /dev/sdb Physical volume "/dev/sdb" successfully created
Next, the volume group needs to be extended to use this additional physical volume:
# vgextend vg_fed2 /dev/sdb Volume group "vg_fed2" successfully extended
At this point the vgs command should report the addition of the approximately 2GB of space to the volume group:
# vgs VG #PV #LV #SN Attr VSize VFree vg_fed2 2 2 0 wz--n- 17.47g 1.97g
Now that the additional space is available in the volume group, the swap logical volume may be extended to utilize the space. First, turn off the swap:
# swapoff /dev/vg_fed2/lv_swap
Next, extend the logical volume to use the new space:
# lvextend -L+1.9GB /dev/vg_fed2/lv_swap Rounding up size to full physical extent 1.91 GiB Extending logical volume lv_swap to 3.84 GiB Logical volume lv_swap successfully resized
Re-create the swap on the logical volume:
# mkswap /dev/vg_fed2/lv_swap
Next, turn swap back on:
# swapon /dev/vg_fed2/lv_swap
Finally, use the swapon –s command to verify the addition of the swap space to the system:
# swapon -s Filename Type Size Used Priority /dev/dm-1 partition 4030456 0 -1
Similarly, the lvdisplay command will list the lv_swap volume as containing 3.84GB of space as opposed to the 1.94GB listed prior to the extension of the volume group and logical volume.
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