On Mon, 27 Jan 2020 23:30:07 +0000, Chris Elvidge wrote:

> On 27/01/2020 22:38, Martin Gregorie wrote:
>> On Mon, 27 Jan 2020 16:22:21 -0500, Dennis Lee Bieber wrote:
>>
>>> Everything I've found in Google implies that one first allocates a
>>> file of known size (cf:
>>> https://help.ubuntu.com/community/SwapFaq#How_do_I_add_a_swap_file.3F
>>> ) which sort of indicates that it does not grow dynamically.
>>>
>> I did say that I don't use swapfiles apart from the default one set up
>> by noobs on my RPi !
>>
>> man 8 swapon   has just about all you need to know, including how to
>> create a swapfile (using dd and /dev/zero - use dd to copy from
>> /dev/zero,
>> specifying the size with something like
>>
>> dd if=/dev/zero of=/swapfile  bs=512 count=3906250
>>
>> should create a 2GB swapfile called 'swapfile' in the root directory.
>> 'count' is the required file size in bytes divided by the block size
>> specified with the 'bs' parameter 512 bytes is the default blocksize
>> given on the 'dd manpage.
>>
>>
>>> That sounds more like Windows behavior, where swap files are given an
>>> initial size, and a maximum size (I think I normally make Windows swap
>>> a fixed size since the OS can allocate a contiguous chunk of disk,
>>> rather than risk a "split" swap).
>>>
>> Could well be: as I said, I've never had to set up a swapfile because
>> all my non-RPIs boxes use swap partitions.
>>
>>> And that brings up the biggest problem with a Linux swap file --
>>> it is likely not going to be contiguous even on disk AND if on an SD
>>> card (or other flash media -- even an SSD can be killed by a system
>>> that is swapping heavily)... Flash memory behavior means that it is
>>> likely every page swap to SD card will trigger a new allocation unit
>>> erase/write cycle and wear leveling means nothing is contiguous.
>>>
>>>
>> man 8 swapon
>>
>> is your friend.
>>
>> And a reminder to those newer to Linux: running "apropos xxxxxx"
>> gives you a list with single line descriptions of every manpage with
>> "xxxxxx" in the manpage's initial NAME section. It has the format:
>>
>> name(s) - one line summary description
>>
>> Find one that looks interesting and then use the command "man progname"
>> to see the manpage for that program or subject.
>>
>> The command "man man" shows you the manpage describing 'man'. Read it
>> to understand the significance of the '8' in my example of reading the
>> swapon manpage.
>>
>>
>>
> I think you'll find Raspbian (Debian?) uses systemd
> dphys-swapfile.service (/etc/init.d/dphys-swapfile & /etc/dhpys-swapfile
> + /lib/systemd/systemd-sysv-install enable/disable dphys-swapfile) to
> compute size and load a swapfile under /var/
> No, it doesn't seem to change size in use. File size is (re)calculated
> at system start.

Does that service recreate the swapfile every time it boots? If so, can
the service definition be edited to change swapfile size if this should
need doing? My RPi isn't running ATM (and isn't in this room) so I can't
easily look at it.


--
Martin    | martin at
Gregorie  | gregorie dot org

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