Re: Swap partition vs swap file

Wakko Warner
2005-07-12T21:39:13Z

Helge Hafting wrote:
> Wakko Warner wrote:
> You don't need to zero out swapfiles. You can fill them with anything,
> even /dev/urandom. Zero-filling may be faster though. A swapfile
> is not zero the second time you use it - then it contains leftovers
> from last time.

I understand this part.

> >So are you saying that if I create a swap partition it's best to use dd to
> >zero it out before mkswap? If no, then why would a file be different? I
> >know there's no documented way to create a file of given size without
> >writing content. I saw windows grow a pagefile several meg in less than a
> >second so I'm sure that it doesn't zero out the space first.
>
> Linux doesn't grow swapfiles at all. It uses what's there at mkswap time.
> You can make new ones of course - manually.

And this part. I've never known linux to grow the swap file. I did try the
sparse one a long time ago. Of course it didn't work.

> >As far as portable, we're talking about linux, portability is not an issue
> >in this case. I myself don't use swap files (or partitions), however,
> >there
> >was a project I recall that would dynamically add/remove swap as needed.
> >Creating a file of 20-50mb quickly would have been beneficial.
>
> You can create 50M quickly - even if it actually have to be written. If
> you can't, don't use that device for swap.

Not all systems can create 50mb in a short time. Especially when the
system/device is under load. Not all systems have multiple disks either.

> Ability to allocate some blocks without actually writing to them is nice
> for this
> purpose, but current linux filesystems doesn't have an api for doing that.
> The necessary changes would touch all existing writeable filesystems, and
> that is a lot of work for very little gain. As they say, you don't
> create swapfiles
> all that often. The time saved on swapfile creation might take a long
> time to
> make up for the time spent on making, auditing and supporting those
> changes.

I hadn't considered this "portability" so I didn't understand at that
point.

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