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KVM/Xen and storage
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KVM/Xen and storage

vedranvedran Member
edited October 2011 in General

I've been browsing through KVM/Xen offers, and I've noticed those usually come with less disk space than OpenVZ offers.

Can someone explain why is that? Disk space cannot be oversold on KVM/Xen and everyone's overselling it on OpenVZ? Or the disk is used for swap? Or something else?

Comments

  • Tradeoff? More security and reliability in exchange for the hard drive space? You're going to get less overselling with KVM/Xen.

  • vedran said: Disk space cannot be oversold on KVM/Xen and everyone's overselling it on OpenVZ?

    This is the main reason. While you allocate X space to someone in OpenVZ, few will use 100% of it, most actually use around 10% of their space, while with KVM/Xen, what you allocate is gone and cannot be shared.

    Hostigation High Resource Hosting - SolusVM KVM VPS / Proxmox OpenVZ VPS- Low Cost Comodo SSL Certificates
  • drmike said: Tradeoff? More security and reliability in exchange for the hard drive space? You're going to get less overselling with KVM/Xen.

    I would have imagined that KVM would come with more disk space because there are less clients to RAM/CPU ratio due to no overselling - this means each customer can have a bigger slice of the disk.

  • That's how it works? u_u

    VMware and Virtualbox can increase the disk size on the fly without allocating it previously.

  • @Ixape said: I would have imagined that KVM would come with more disk space because there are less clients to RAM/CPU ratio due to no overselling - this means each customer can have a bigger slice of the disk.

    That's exactly what I'm thinking. If the same nodes are used for KVM and OpenVZ, they'll be able to put less clients on KVM, meaning it will be less clients per total storage. Yes, OpenVZ storage is oversold, but how much?

    According to @miTgiB, selling 500% of total storage space should be quite safe, if you monitor free space and make sure you're able to quickly add more if needed.

  • @Ixape If you get less VMs on a box, shouldn't you also pay more for it? Then it would put the same xen/KVM plans on a different price point as OpenVZ plans, which makes comparison not as useful?

    Blog at LowEndBox.com.

  • LowEndAdmin said: @Ixape If you get less VMs on a box, shouldn't you also pay more for it? Then it would put the same xen/KVM plans on a different price point as OpenVZ plans, which makes comparison not as useful?

    Totally. For me it doesn't make sense to have empty disk space on a server - you may as well let the customer use it and give them a good experience.

  • But you are paying more, look at this offer: http://www.lowendbox.com/blog/tinyvz-15year-128mb-openvz-vps-in-kansas-city/

    That's just an example, KVM/Xen offers usually look like that. Higher price, everything is the same except for disk space.

  • 500% overselling?

    500%?!?!?

    Maybe twice but 500%?!?

  • Keep in mind that disk io load on kvm and xen is much higher that openvz as all the kernels need to be read and not just one.

    YourVZ.com

  • @JustinBoshoff said: Keep in mind that disk io load on kvm and xen is much higher that openvz as all the kernels need to be read and not just one.

    LOL, the kernel is just read one time... in the boot

  • JustinBoshoffJustinBoshoff Member
    edited October 2011

    OK sorry maybe "kernel" was the wrong word, what I mean is 30 base systems have more io needs than 1. Sorry!

    YourVZ.com

  • @JustinBoshoff It's a fairly moot point. If I have two identical nodes as far as hardware, I'm going to get much fewer containers on a KVM node then OpenVZ, so even if KVM requires more IO, with fewer users requesting the resource, you'll not see much change in performance, and reality shows much higher performance quite often.

    Hostigation High Resource Hosting - SolusVM KVM VPS / Proxmox OpenVZ VPS- Low Cost Comodo SSL Certificates
  • FranciscoFrancisco Top Provider

    @drmike said: Tradeoff? More security and reliability in exchange for the hard drive space? You're going to get less overselling with KVM/Xen.

    Not really.

    Good ol' thrustVPS had their like $4/m 1GB KVM's and they ran like ass after a couple weeks just because they oversold it so hardcore. They claimed it was 'BETA' but never addressed it.

    With the harddrive market being boned I expect a lot less space for future plans for a while for many providers as they try to work within their budgets. I know for us that minus the sales round next week we're not doing anything large any time soon just because 1.5TB+ drives are through the roof.

    Francisco

    BuyVM - Free DirectAdmin, Softaculous, & Blesta! / Anycast Support! / Windows 2008, 2012, & 2016! / Unmetered Bandwidth!
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  • kiloservekiloserve Member
    edited October 2011

    With OpenVZ, you can oversell diskspace to a certain degree without a negative hit on server performance since it's pretty much native disk access with little CPU assistance if you have good hardware RAID.

    However, on KVM, disk I/O is actually emulated with assistance from the CPU and VT/VT-D.

    That KVM I/O emulation layer eats up CPU cycles so the more disk activity you have, the more CPU cycles you eat up.

    If you sell too much disk space on a KVM node and the user actually utilizes it, the performance impact on the system will be much higher than if it was OpenVZ.

    Overselling can be done regardless of the virtualization platform...it is not an issue with the technology but rather it is provider dependent.

    And truthfully, responsible Overselling is a good thing...it keeps the provider out of the dead pool while giving the consumer a great price on an excellent product.

    Overselling is perfectly acceptable as long as the provider doesn't overload the server. Good providers know what performance level they want to provide and stop overselling when they hit their performance mark.

    Why would it matter if a provider Oversells as long as the server is consistently fast?

  • JustinBoshoffJustinBoshoff Member
    edited November 2011

    @miTgiB
    Moot point is one of those phrases that once had a firm and well-understood meaning, but no longer does. It was just as you say: a matter that was uncertain or undecided, so open to debate.

    KVM's overhead is about 10% to 20% depending on what the vm is doing, OpenVZ's overhead is about 1% to 3% so in theory you could have 10 times the density on Openvz.

    So I would think most providers will run the same spindle count but smaller disks to keep the costs down as per node you will have less income.

    YourVZ.com

  • JustinBoshoff said: @miTgiB Moot point is one of those phrases that once had a firm and well-understood meaning, but no longer does.

    The phrase has not changed. People are simply less intelligent, more belligerent, and are more likely to argue for the sake of hearing their own voice.

  • @Aldryic
    That is so true. Most of the Dumb Asses "like me" make them selves feel important by doing so! LOL

    YourVZ.com

  • @Aldryic said: People are simply less intelligent, more belligerent, and are more likely to argue for the sake of hearing their own voice.

    Wait, who are you calling a dumb ass?

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  • ^_^

  • JustinBoshoffJustinBoshoff Member
    edited November 2011

    @miTgiB
    My self, but anybody is welcome to join me in the "Dumb Ass Pool" if they feel like it. LOL

    YourVZ.com

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