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Showing posts from July, 2013

SSD overprovisioning in vSphere

SSD vendors like Samsung or Intel often provide tools to reserve some space on your SSD that can be used by the internal algorithms for better wear-leveling, longevity, garbage collection, and performance. My own Samsung SSD comes with a tool to do that on Windows, for example.
But on vSphere, you'll have to do that manually: I see two possibilities:

don't format the SSD "full" but instead use "partial", and specify only the amount of the disk that you want to use. The rest is left unpartitioned, and therefore unallocated. You can only do this if you haven't formatted the SSD as VMFS yet. If you have, this second possibility can still help you:create a thick-provisioned lazy-zeroed VMDK on the SSD-backed VMFS, either with vmkfstools or by creating a dummy VM. Those blocks are allocated, but remain unwritten. You're not going to attach the disk to a running VM, so they won't ever be written to. This effectively reduces the amount of blocks that ca…

dd-wrt refusing new configuration: out of NVRAM space

Another post documenting a small issue just so I can google it later: one of the Cisco/Linksys WRT160NL devices (both running DD-WRT) in my environment stopped accepting config changes through its web interface. It happened to be the one I'm using as a NAT gateway and DHCP server, not the one just functioning as access point.
A config backup appeared to be full of UPnP rules. Those are port forwarding rules that have been asked for by applications on NATed stations that want to be reachable. Think Skype amongst many others.
Clearing the UPnP list solved the problem: click "NAT/QoS", then "UPnP", then "Delete all". Apparently the UPnP list filled up the available NVRAM space, which broke all subsequent config changes.