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.


--

Chris Elvidge, England

--- SoupGate-Win32 v1.05