20 pts.
Virtual Machines
We have a need to run numerous virtual servers from a NAS/SAN. The questions is, which should we choose? THe product we support (Citrix XenServer) supports either for backend virtual system storage and can run VM's from either. Currently, our needs are small. We have about 10 virtual servers that we want to migrate to a centrally accessable storage box so we can take advantage of XenServer's host pooling options (which allows us to migrate the management of a VM from one server to another while the systems are running, without performance issues, as well as system failover and automatic performance tuning.) The host servers are connected via GbE and the current config is DAS on RAID 5. WE need either a NFS based NAS or software iSCSI SAN to share this between the FC or SCSI HBA's. We have three different database servers but they only service 10 employees per department in 3 departments (each one has their own domain and DB server.) I understand that SAN has the potential to be faster but they're significantly more expensive. What are the trade-offs between NAS and SAN? What about DR/replication? Any insights are helpful!

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I believe that the answer would depend on how much speed you need. Most NAS that I have seen are copper ethernet and you can get up to GB speed. whereas a SAN can be ordered with FibreChannel connections to have fiber speed to the SAN itself which can make it much faster than GB speeds depending on how much money you want to spend. How mission critical are the apps you are going to be running? That can also answer your question but it usually depends on your finances.

another question is how much storage do you need? A SAN scales to many terabytes easily whereas if you used NAS you would have many NAS devices added to your network and once you started to scale up this would not be practical.

— Yuval —

A big difference betweens SAN and NAS is that a NAS provides its own filesystem, while a SAN provides block-level access. This makes it better for things like databases. SAN also allows you to do DR and load balancing by moving the VMs from server to server as needed. There’s a bit more on the overall benefits of SANs at’s Hot Spot SAN tutorial (there’s also a third part, going over SAN features and implementation details, that should be up in a couple days).

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  • Isdpcman
    There are some physical issues we have with this. If we go with SAN we can only use an iSCSI initiator as the host VM servers must be able to share this resource. They cannot use an HBA for this. While we will undoubtedly have SQL Server needs they will be small (e.g. 30 - 80 users tops) I will check out the link you have, thanks. This is new territory for me and I want to make the right choices for our needs. I appreciate your advice!
    20 pointsBadges:
  • Rcp123
    The performance differences between NAS and SAN start to blur at the high-end. There are NAS devices out there that have fairly high performance numbers. Several come with multiple GigE or even 10GigE that can then be bonded together. Our own NAS has sustained over 70K IOPS . This is one server, not several. NAS can also scale fairly high -- into the PB. Both NAS and SAN storage offer replication and DR. Though at the lowest end, you may be hard pressed to find a SAN solution that provides this and may have to resort to purchasing something 3rd party. Most NAS will offer some type of the very least to another NAS box of the same type. Traditionally NAS has been simpler to administer once you set it up. SAN has had the stigma of being difficult but I think that's no longer true, especially with the newer storage devices available out there. I think in either case, you should think about how you plan on scaling this service. If this is just the beginning, and you think you'll be growing a lot, you may want to consider choosing backends which allow you to spread the load across all storage spindles non-disruptively. This allows you to keep growing your front-end compute w/o taking the downtime to upgrade your back-end storage should it ever become your bottleneck.
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