SAN Storage For Disaster Recovery Solutions
Posted on 31.Jan 2010 by Ray Heffer in SAN Storage
It’s Monday morning and you arrive late at the office thanks to the trains being delayed yet again. At that particular moment in time as you grab your morning coffee, several hundred users have already logged in and started launching their email client, web applications, and a myriad of documents and spreadsheets. So far this sounds like any other morning, but what I didn’t mention was the fact that just 30 minutes before you arrived at the office, a water from a pipe in the ceiling started to leak into the rack containing your SAN’s disk array.
VMware ESX 3.x Snapshots
Posted on 12.Oct 2009 by Ray Heffer in VMware, VirtualisationSnapshots are a fantastic way of providing a quick and reliable method of rolling back the state of a virtual machine, should something go astray following an patch or update. VMware VCB also uses virtual machine snapshots to quiesce the VM prior to taking the backup data.
However, in larger environments where there may be tens or hundreds of VMware ESX servers, snapshots can also be a pain in the backside if there is no control over who is using them. Why? Because snapshots work by creating a delta VMDK that records the changes in blocks, a process called copy-on-write (COW). Over time the delta VMDK file will grow, and depending on the level of I/O within the VM it could grow faster on some virtual machines and not others.
The danger only presents itself if the datastore where the VMDK resides reaches it’s capacity. When this happens, virtual machines that are not thin-provisioned should continue to run with no problems, but think about these situations:
1) You have other virtual machines in the same datastore using snapshots.
2) You have one or more virtual machines on thin-provisioned disks.
3) You have powered off virtual machines, that need to be powered on.
In all of the above scenarios if the datastore is full then the affected virtual machines will be suspended (paused). Virtual machines with thick-provisioned disks will continue to operate as the VMDK already has the full allocated of storage space available. Virtual machines that are powered off, and need to be powered back on will fail as they won’t have enough disk space to create the virtual swap file.
