Should Linux Swap to a Virtual Disk In Storage (VDISK)?
To explore swapping to VDISK,
first we need to look at what happens when
your Linux guest swaps.
Next we examine administrative characteristics of VDISK.
Finally, we look at considerations
for storage-rich and storage-constrained systems.
Note that while the phrases "swap disk" and "swapping"
are often used in describing Linux,
we should remember that
Linux does not
swap process spaces in the traditional sense, but instead
paging based on least recently used algorithms. For historical
reasons, the term swap is still shown and used, but it
refers to paging in Linux.
swap is also used to help delineate paging at the
Linux level (swap disk), in contrast to paging at the z/VM
What Happens When Your Linux Guest Swaps
When a Linux guest swaps (pages), it is doing I/O to its swap DASD
in support of the virtualization of its memory.
These factors combine to produce
time and decreased transaction rates for Linux.
Further, they have an effect no matter what the
target device is.
- The Linux guest is incurring guest path length and I/O
latency because it is doing swap I/O.
- z/VM is incurring Control Program path length and I/O
latency on behalf of the guest when it does the I/O to the
swap DASD on behalf of the guest.
Administrative Aspects of VDISK
A VDISK makes a very
attractive swap device from an administration point of view.
It is easy to set up a VDISK, in fact, usually your z/VM
system administrator does not need to be involved.
The CP DEFINE command or a user directory statement can
be employed to set up a VDISK.
Performance Effects of Swapping to VDISK
Swapping to VDISK has potential for
improving system performance. It can also be detrimental
to performance. Following are the considerations one
must keep in mind.
Because pages that back the virtual disk blocks of a VDISK
are only created if the virtual disk block is created.
It can be advantageous to define multiple VDISKs as swap
space instead of a single large VDISK. You would also
configure Linux to use the first VDISK higher in its
hierarchy. This will allow Linux to reuse freed disk blocks
on the first VDISK before moving onto the next one. This
may lower the total number of pages representing the VDISKs
to be backed by z/VM.
If your system has plenty of real storage,
you can reduce latency in Linux swap I/O by putting the Linux
swap volume on
a VDISK. The VDISK is a very fast I/O device when its
implemented as pages in an address space,
can remain memory-resident.
Note that using a VDISK does not change the other
I/O behaviors of
Linux or z/VM.
Linux must still form and emit I/Os to read and write the
swap extent, and
z/VM must still perform these I/Os for the guest.
Though the pages backing the VDISK blocks
themselves are not created until referenced,
the architected control blocks for dynamic address translation
(page tables and segment tables, mainly) are created at the time the
VDISK is created.
Further, these control blocks are not
The VM storage management steal processing has a
hierarchy of pages that it uses in trying to select the most
In that hierarchy, normal idle guest pages are
chosen for steal more readily
than VDISK pages (a
virtual disk in storage page is implemented as
a system utility space and
therefore is given preferential treatment).
As Linux guests go idle,
this hierarchy might have an undesireable effect.
Linux will determine an
unused page and move it out to its swap disk (a VDISK).
If the Linux guest
goes idle, VM storage management will
steal more aggressively from the guest pages (pages Linux decided
it needed) and less aggressively from the VDISK
(pages Linux decided it was safe to page out). This is counter
to what good performance would dictate.
You can tell whether you are storage-constrained by using
Performance Toolkit, as illustrated in the table below.
Option 3A, PAG selection
Option 13, DEV selection
Option 11, CHAN selection
Back to the Performance Tips Page