Re: System performance using swap partition vs. swap file

[Rick Stevens <rstevens vitalstream com>]

Mark Knecht wrote:
> Rick,
>    You made my Linux day to think that I might have provided a good idea
> back to you after all of your work here! <broad smile> Unfortunately, it
> wasn't my idea personally, but I don't mind being the messenger.
>
>    My current swap partition is a 1GB partition, so I shouldn't have any
> problems with size. The machine currently has 256MB in it, but I might
> go to 512MB one of these days, so I chose 1GB to get ready for that.

Good idea.  I have two 2GB swap partitions myself, but only have 512MB
of RAM.

>    I don't understand the use for multiple swap partitions, but I'm
> guessing it's either a server or SMP issue? Possibly really large memory
> machines, say 4GB or DRAM, need 8GB or swap, so you have to break it
> into pieces?

That's it.  It goes back to ye old days of having a signed long as the
file offset variable.  The biggest signed, 32-bit value is
2GB.  Why signed?  Because lseek()'s prototype was:

	long lseek(int filedes, long offset, int whence);

so you could "lseek(fd, -256, SEEK_CUR)" and move back 256 bytes in
the file.  This is also the cause of problems with some programs
using files >2GB (e.g. apache).

lseek now uses "off_t" types, which are "long long" ints, giving you an
unsigned 64-bit integer.  Newer programs know this and have been
recompiled with the bigger value.

Details, details!

>    Anyway, I'll have to see if any of my books cover making and
> installing swap files.

Make your partition and mount it.  Create a contiguous file of the
size you want filled with zeroes (for example, 2GB):

    # dd if=/dev/zero of=/mountpoint/swapfile1 bs=1M count=2048

Then "swapon /mountpoint/swapfile1" to activate it.  You can also
add a line like:

    /mountpoint/swapfile1   swap   swap    defaults        0 0

to your /etc/fstab and the system will activate it (via "swapon -a")
on the next boot automatically.

>    I can't see the need for ext3 on a swap file.I know you didn't really
> suggest it, but the possibility crept in the way you responded.

No, I just meant that the label is stored in the filesystem's header.
swap filesystems do not support labels.
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- Rick Stevens, Senior Systems Engineer     rstevens vitalstream com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-            We look for things.  Things that make us go!            -
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