Database Reference
In-Depth Information
has been configured to avoid paging and that the VMs have been configured and are
running on ESXi hosts that are properly sized to avoid swapping.
vSphere provides the ability to locate the ESXi host swap file location on SSDs inside
the ESXi host. This is called “host-local swap.” Although this is an option, it should be
viewed as a failsafe option used as a stopgap measure until the memory exhaustion issue
can be resolved. If you are implementing host-local swap, keep in mind vMotion times
will take longer because the swap file must also be migrated across the network from
the source host to the destination host. This also means you have expensive SSDs in
each host that are not being optimally utilized given that memory contention should be
very unlikely in a properly designed environment.
We have seen some customers design their ESXi host swap files on shared storage and
present this to all their ESXi hosts (for example, implementations using replication
technologies and not wanting to replicate the virtual machine swap file). A word of
caution around this design: If the LUN containing the vswp files fails, then all virtual
machines configured to use this LUN have just had their vswp files ripped out from
under them, and this could potentially lead to guest operating system failure. If you are
considering relocation of your vswp files (for which there are valid reasons), run them
on a reduced number of LUNs or a particular LUN per host. We do not think it is a good
economic use of SSDs to have swap files on them; plus, if your VMs have large RAM
allocations, you could find a situation where a VM can't power on because of
insufficient local SSD space for the swap file if there isn't a big enough reservation.
Once you're swapping, it's already too late. A LUN expands the failure zone to all
virtual machines having vswp files residing on that LUN. Consider two LUNs, at a
minimum, because this reduces complexity while reducing risk.
Note
To change the default location of the virtual machine swap file, see this
VMware KB article:
http://kb.vmware.com/kb/1004082
.
Keep in mind
your failure zones and operational impact when making this change.
Large Pages
By default, VMware vSphere enables large pages. Mem.AllocGuest.LargePage is set to
1 out of the box, which means it is enabled. By default, ESXi will back all memory
requests with large pages. These large pages are broken down to 4KB in size to support
transparent page sharing (TPS). This is done to maximize the use of the precious
translation lookaside buffer (TLB) space and to increase performance.
Microsoft SQL does support the use of large pages, and beginning with SQL Server
2012 this is enabled by default when the account running sqlservr.exe has been given the
