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I'm using a Raspberry Pi 4 with 4GB and one with 8GB for docker/ pi-hole. Currently I use the default settings for the swap file and know with the older Raspberry Pi modells with less RAM, the suggestion was to change the swap file to a HDD. I have old 2.5" drives but 1.) Do I still need a swap file or could I disable it? 2.) how to I measure the performance

Chris
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  • disable swap is not recommanded as, when the OS decides it needs to swap, a non existing swap will result in a non recoverable failure (in most cases). Without Swap, the OS will call the OOM killer which tries to get some free ram... but ists not a feature intended for that problem... its just for a stalling system to keep logging, etc... up as long as possible (to my understanding) and chances are: there is no free ram, or it kills something you need for your system providing ists services – schnedan Feb 08 '22 at 12:36
  • "its just for a stalling system to keep logging, etc... up as long as possible" -> No, that's not really the case. Nor will it save you from the OOM killer because swap is also finite. The point of it is to allow you to run more software at one time. When RAM usage reaches a certain level, the kernel will begin to "swap out" the data that has/is being accessed the least to swap (which can have various forms including a file, a partition, and compressed normal mem), on the premise that wherever it is, it is slower to access than RAM. Point being, it is not an emergency system. – goldilocks Feb 08 '22 at 14:22

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You can run without swap, but it is not advisable. If you are short on memory, either because you don't have much RAM or because you have memory-intensive processes running, you will probably need swap.

But then, how large should your swap space be? As a first idea, twice the size of RAM if RAM is less than 2 GB or size of RAM + 2 GB if RAM size is more than 2 GB. But that's a starting point.

If you really want to know, use vmstat. In the column swpd, you can see how many kB swap you're using. If swpd is consistently 0, you may try to run without swap, if you really want. If it is non-zero, swap is being used. And then it was good that you have it.

The next question would be whether you should put the swap on an external drive. The reasoning behind this is that using an external drive saves writes to the SD card and that will prolong the life of the card. In general: if your swap is heavily used, you will use an external drive. If there is little or no activity, you might leave the swap on the SD card. How do you see whether the swap is actively used? Again, vmstat will show you:

  • If the columns si and so are non-zero, you have swap activity.
  • A wildly varying value in the column swpd is also an indication of swap activity

And now for some examples.

My DNS server has quite sufficient memory to run bind9. This is a sample of the vmstat:

ljm@phi:~$ vmstat 60
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0      0 544664  47520 241992    0    0     0     1    1    1  0  0 100  0  0
 0  0      0 544664  47520 241992    0    0     0     0  337   35  0  0 100  0  0
 0  0      0 544664  47520 241992    0    0     0     0  378   41  0  0 100  0  0
 0  0      0 544664  47520 241992    0    0     0     0  366   37  0  0 100  0  0
^C

This means that the pi could probably do without swap

Pi is my mail server.

 ljm@pi:~$ vmstat 60
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0  16096  11744  57116 314056    0    0     4     4    3    3  4 24 54 19
 1  0  16096  11504  57132 314088    0    0     0     2 1245 1892  0  1 99  0
 1  0  16096  11184  57504 311892    0    0    15     6 1265 1938  3  4 93  0
 0  0  16096  12012  57872 310632    0    0     1    77 1304 2040  7  9 81  4
 1  2  16096  17468  58072 305464    0    0  1169   119 1724 2760  6 14 65 16
 3  0  16096  15088  56792 308452    0    0  4390    10 3351 6941 10 50  0 39
 3  0  16096  10376  58140 306128    0    0  5212     6 3267 5848 13 45  0 42
 1  0  16096  17972  58172 304368    0    0  1515     4 2115 4312  4 22 61 13
 1  0  16096  15740  58220 306520    0    0    35    10 1255 1912  3  2 93  2

You see swap > 0. So a swap space here is also being used. However si and so are zero and swpd is consistently 16096. So, your swap could be on the SD card.

Last example I plucked of the Internet:

procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 3  1 244208  10312   1552  62636    4   23    98   249   44  304 28  3 68  1  0
 0  2 244920   6852   1844  67284    0  544  5248   544  236 1655  4  6  0 90  0
 1  2 256556   7468   1892  69356    0 3404  6048  3448  290 2604  5 12  0 83  0
 0  2 263832   8416   1952  71028    0 3788  2792  3788  140 2926 12 14  0 74  0
 0  3 274492   7704   1964  73064    0 4444  2812  5840  295 4201  8 22  0 69  0

You see so is non-zero. And swpd varies wildly. You have a RAM shortage ans swapping helps to virtually increase the memory. Because swap is so active, it would be wise to use an external drive for swap.

Ljm Dullaart
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  • While this is mostly a decent answer, some of your assertions implying that if swap is used means it is required are a bit strong; if swap is enabled, it will be used long before you actually run out of RAM and in that sense need it. Search online for "linux swapiness," the primary setting that controls how it is triggered. It is not uncommon to adjust this as some people feel swap by default is used too eagerly to no benefit (which is perhaps a bit too tinkery, but makes the point that it happens in normal, lots of RAM still available situations). – goldilocks Feb 08 '22 at 15:49
  • Here's a good article which also directly addresses the errors of some less good explanations: https://www.howtogeek.com/449691/what-is-swapiness-on-linux-and-how-to-change-it/ ...Unfortunately a bit ad heavy. Swap is a bit like climate control. That your AC comes on above a certain temperature does not mean your house was about to catch fire or someone would die from heat exhaustion, etc. There are conventions about room temperature, personal preferences, cost-benefit analysis etc. – goldilocks Feb 08 '22 at 15:55
  • To be honest, my last ordeals with the OOM-killer where on AIX 4 together with SIGDANGER problems. – Ljm Dullaart Feb 08 '22 at 16:22