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How busy is your Xen host?

Posted on | October 22, 2007 | No Comments

Several people have written to me asking how to determine how ‘busy’ their Xen hosting provider’s servers have become. I just love new buzzwords, ‘busy’ now indicates how many VPS accounts that a web host packs on a single machine.

To scope out a Xen host, you need to ask them some questions about their processors, drives and network hardware. Xen does not ‘burst’ resources to any single VPS. By default, Xen gives each VPS on the server an equal share of resources, proportional to how many CPU’s the VPS is assigned.

Once your VPS has booted, Xen will not take away from your assigned memory. If you booted with 256 MB, you will keep 256 MB. Nobody else can use your RAM unless the host ‘shrinks’ your VPS to free up memory in order to give it to someone else. You’d notice this quickly. If you want to make sure that you buy a VPS that really delivers everything that Xen has to offer, here’s some questions to ask your host and explanations of why to ask them:

  • What kind of processors are in your Xen servers, how many are there?

You want to hear “2 way dual core (something)” , ask them for a link to the type of processor and motherboard that they’re using. This is not “top secret” information. You want to see something (relatively) new.

  • Do you pin your VPS’s on certain cores to balance things out, or do you just rely on Xen’s credit scheduler to give equal share?

A Xen host can give your VPS (in theory) up to 32 CPU’s. Most give out 1, 2 or 4 depending on what you pay. Xen offers them the ability to ‘pin’ your CPU’s on a specific core of a specific processor on their side. Some hosts use both cores of one processor to power lower end packages, reserving access to the other two cores on the other processor for “more power” upgrades.

There is (really) no good answer to this question. If they are able to demonstrate to you that they know how (and can) go beyond the default credit scheduler, its a good sign.

  • What kind of drives are used to give your VPS servers their block devices?

This is a very important question. You want (ideally) SAS or SCSI backed storage. The faster their drives spin, the faster everyone’s VPS will work. You may have a super fast dual core Opteron CPU, if 30 VPS users are sharing a single IDE drive, the whole rig will crawl. Services like MySQL must have fast disk access, especially for busy web sites.

Many hosts use network storage. Ask your perspective host if they use network storage and what kind. Your most curious about the connection between the Xen server and the storage. You want fiber, not copper. iSCSI over gig-e is going to be horribly, horribly slow and prone to breakage. You should also ask them if the swap block device is local or also residing on network storage. It is very senseless to swap to a network block device.

Make sure that the host is using logical volumes (LVM) or real partitions to back your VPS drives. Some still use file backed block devices (they create a file on their side and make it a file system, which becomes your hard drive). This method is super slow and rather dangerous, it makes you more prone to file system errors in the event of heavy usage.

A good host will have all local disks, or fiber connected network storage with a local disk that provides swap for the VPS accounts. This is also not “top secret”, feel free to ask.

  • How many network cards are in your Xen servers? How many VPS’s share an Ethernet bridge at any given time?

The most popular network configuration for Xen (for hosts) is to use one network card and enslave the card to something called a bridge, this allows many virtual machines to share a single physical network adapter. If your host has a server with 16 GB of ram, they can sell 30 – 40 virtual machines using that server. This means, 30 – 40 virtual machines can (potentially) wind up sharing one network adapter. Yikes!

Make sure that your host uses multiple network adapters. Ask them to explain to you how they audit to ensure that they keep network traffic balanced over multiple adapters and bridges. Bandwidth usage per VPS is rarely ever static, a good Xen host will watch who is using what and make sure that the heavy hitters are balanced over multiple network devices and bridges. This helps ensure that you get the full 10 or 100MB up-link that you pay for. If the host is not advertising up-link speed, make them specify the up-link speed before you spend money. Many will say “you get 400 GB/ Monthly”, well – that’s no good if your VPS appears to have a dial up connection to the Internet :)

Most good Xen hosts will have traffic shaping in place allowing them full control over the up-link speed of each virtual machine.

  • What kind of controls do you offer so that I can re-boot or repair my VPS remotely without waiting for your support department?

Unless the host is using a proprietary version from XenSource, by default, no web based controls will exist. Good Xen hosts will develop some kind of portal to allow you to re-boot your VPS, repair your file systems or (sometimes) re-load your operating system. Ask to see a demonstration of this system prior to spending your money.

  • Do you manage backups? If so, how?

Many hosts will tell you that backups are your responsibility. This is a perfectly acceptable answer, but you need to know right off the bat if you should have your own backup scripts in place.

Other hosts use mirrored storage that permits them to take snap shots of your file system without pausing your VPS. Some use snap shots without mirrored storage, this requires that your VPS be (at least) paused during the backup cycle. No matter what the host does, you should also make your own backups, that is a no-brainer.

Anyone answering sales chats should know the answers to these questions, if they don’t – consider it a warning flag. How can someone sell you something if they really have no idea how it works to begin with?

There are other questions to ask, such as how they build and pair their hypervisor to their dom-0 kernels then to your kernel. A 64 bit hypervisor with a 32 bit PAE dom-0 kernel is a very efficient configuration, permitting both 32 and 64 bit guests. I would not expect a sales person to know this, it would be a big plus if they did. If your a geek and appreciate knowing the details of their configuration toss out the question.

If your using a Xen VPS and feel that its really slow, its not too late to ask your host the above questions. Knowing the answers to the above can usually give you a good indication of how ‘busy’ things really are and why. Prevention is better than a cure, so make sure you ask these questions before spending money :)


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