最後更新: 2020-07-15

介紹

 

將硬盤空間當作 RAM 使以, 以此防止系統出現 "out of memory" 情況

"out of memory" 會觸發 Kernel "OOM killer mechanism"

目錄

• 建立 swap (mkswap)
• 啟用 swap (swapon / swapoff)
• Priority 小實驗 - 自動 priority
• swap on file
• fstab swap settings
• 某 Procces 的 swap 使用情況
• 把 swap 推回 RAM
• Check What's In Swap
• Notes
• More

---

建立 swap (mkswap)

 

mkswap -L [label] [partition]

i.e.

mkswap /data/swapfile

Output:

Setting up swapspace version 1, size = 200700 KiB
no label, UUID=60ef3f84-a5e4-4ad4-b867-0c8ca39c6ba5

 

---

啟用 swap (swapon / swapoff)

 

/sbin/swapon [-v] [-p priority]  <specialfile/device>

顯示當時 swap 情況

swapon    # 不帶參數時

NAME       TYPE      SIZE  USED PRIO
/dev/sdd6  partition 7.8G 77.7M   -2
/dev/zram0 partition   2G  512K    1

Opts:

-a                             # All devices marked as  "swap'' in /etc/fstab

-e                             # 與 -a 一起用, 功能是略過不存在的 swap device

-p, --priority <prio>   # 0 (Lowest) ~ 32767 (Highest); 在 fstab 內用 "pri=value" 設定它

                                # 當沒有設定時, 就會自動指定一個遞減 -n 值 (-2, -3, -4 ...)

-s                             # Display swap usage summary by device ( 相當於 cat /proc/swap )

# 單位 byte
Filename           Type            Size    Used    Priority
/var/swap          file            102396  0       -1

 * PRIO = -1 => Default Value

Column:

• swap file name
• the type of swap space
• the total size
• the amount of space in use (in kilobytes)
• the priority (useful when multiple swap files)

 

---

Priority 小實驗 - 自動 priority

 

swapon

NAME       TYPE      SIZE USED PRIO
/dev/sdd6  partition 7.8G   9M   -2
/dev/zram0 partition   2G   0B   -3

swapoff /dev/sdd6 && swapon

NAME       TYPE      SIZE USED PRIO
/dev/zram0 partition   2G 1.2M   -2

swapon /dev/sdd6 && swapon

NAME       TYPE      SIZE  USED PRIO
/dev/sdd6  partition 7.8G    0B   -3
/dev/zram0 partition   2G 12.5M   -2

swapoff /dev/zram0 && swapon -p 1 /dev/zram0 && swapon

NAME       TYPE      SIZE USED PRIO
/dev/sdd6  partition 7.8G 4.8M   -2
/dev/zram0 partition   2G 6.8M    1

swapoff /dev/sdd6 && swapon /dev/sdd6 && swapon

NAME       TYPE      SIZE  USED PRIO
/dev/sdd6  partition 7.8G    0B   -2
/dev/zram0 partition   2G 15.6M    1

 

---

swap on file

 

1. 建立一個2GB 的 swap file

dd if=/dev/zero of=/swapfile bs=1M count=2048

Notes

不應用 fallocate, dmesg 會有

swapon: swapfile has holes

2. 保護它

chown root:root /swapfile

chmod 0600 /swapfile

3. format 它

mkswap /swapfile

output

Setting up swapspace version 1, size = 2147479 kB
no label, UUID=56adce50-fd10-4a36-ac81-550d891c1eb1

4. 使用它

swapon /swapfile

# checking

dmesg | tail

... Adding 2097148k swap on /swapfile.  Priority:-2 extents:2 across:3334140k SSFS

free -m

 

---

fstab swap settings

 

/etc/fstab:

# swap settings
UUID=ccda3242-1561-402a-baf6-ea13b883fa2b     none   swap   sw      0 0
UUID=/dev/sdb3                                none   swap   noauto  0 0
/swapfile                                     none   swap   pri=10  0 0

 * pri 值愈高就會愈被使用先

 * 多個 Swap 的好處: The kernel itself can stripe swapping on several devices (if you just give them the same priority)

 * All devices marked as "swap'' in /etc/fstab are made available, except for those with the "noauto" option.

 * the 'sw' option = placeholder = 沒有其他意思

不確定 Device 是否存在時

# SWAP
UUID=ID       none swap nofail,x-systemd.device-timeout=15s 0 0

 

---

某 Procces 的 swap 使用情況

 

By /proc

# 找出 pid

pidof kvm

4819

# 找出 swap 的用量

grep VmSwap /proc/4819/status

VmSwap:    12644 kB

By smem

dnf install smem

smem -t -k          # -t, --totals; -k, --abbreviate = Show unit suffixes (K, M)

  PID User     Command                         Swap      USS      PSS      RSS
  456 root     /sbin/udevd -d                472.0K        0     1.0K    44.0K
 1660 root     /sbin/udevd -d                468.0K        0     1.0K    44.0K
...

例子: 有 process 嚴重 swap

 

 

---

把 swap 推回 RAM

 

* You must have enough physical memory available on the system

swapoff -a & swapon -a

 

---

Notes

 

 * You should not use swapon on a file with holes

 * Swap over NFS(not allowed by the kernel/need kernel patch), btrfs(cow) do not work

 

---

More

 

• http://datahunter.org/memory