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07-11-2011, 05:03 PM #26Aspiring Evangelist
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I think what you've seen is various providers having problems with SANs that haven't been thoroughly tested by their manufacturers under high-load conditions that typically exist with multi-tenant environments. Firmware bugs are to blame.
However, this is not to say that a SAN is a bad idea, nor that there are no SAN vendors that make quality products. There are plenty that make great products that "just work".
Local storage is a much simpler concept since it is not shared among multiple machines and lacks the availability features while modifying the storage parameters.
This is a list of features that you typically don't find in local storage:
- LUNs (a virtual disk volume that is a "chunk" of storage in a RAID)
- thin-provisioning
- 2 active/active controllers with replicated caches
- 4+ I/O paths (all active)
- online firmware updates
- online expansion of disk shelves
- online LUN and RAID expansion and contraction
- deduplication
- compression
- asynchronous and synchronous replication
- online LUN mapping to one or more servers
- shared LUNs for clustered servers
- storage tiering (automatic moving of data to slower/faster storage based on usage stats)
- online RAID migration
- hot-swap of every part including power supplies, drives, controllers, and even disk shelves
- snapshots
Many SANs provide the ability to handle just about every storage scenario known to exist, while maintaining availability. The SANs that have had issues due to bugs have create a black eye for various cloud providers and so they get a bad rap, but the number is fewer and fewer now that virtualization has pushed these devices to their limits and manufacturers have had to step up and fix the issues.
You are also seeing a shift in economics, where SANs that have all of these features have been extremely expensive due to the development and testing needed to be sure all of these features work properly in 99.999% uptime conditions, but now you have cheaper and cheaper SANs that are in competition. These cheaper SANs don't have near the bench time and real-world stress-test time that the more mature SANs have had and so you see strange problems showing up that nobody ever saw before.
Sometimes, the problem isn't with the SAN at all, but with the hard drives themselves. We've seen a rare problem that occurred where Western Digital had a firmware bug in their drives and the SAS signal voltage dipped too much when many drives were connected to the same power supplies and were all being worked hard. One would think it was related to the power supplies, but it wasn't... it was a software bug in the drives themselves that caused the voltage to dip out of compliance of the SAS standard. This caused multiple drives to appear as failed. Not good. But this could also happen in a local storage scenario with enough drives.
It all depends on the environment, budget, uptime requirements, and features, but SANs have a very important place in the world, just as local storage does. If there are situations where none of the above features are required, and the availability risk is sufficient (over sustained periods of time... such as 5+ years), local storage is a great solution.
EricGenesis Hosting Solutions, LLC (genesishosting.com)
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07-11-2011, 08:17 PM #27Away
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Eric,
I just wanted to commend you on a very well thought out and informative post. I think you hit the nail on the head here.
The thing that should be conveyed by clouds using SAN is depending on the setup/software you can truly offer failover where with local storage it isn't really possible (I could be mistaken but this is just my observation). We also need to remember the WHT market which is typically (not exclusively) pretty thin on margins. If you are willing to pay for a cloud server with enterprise SAN storage you can find it from a number of providers out there that target enterprise type customers who have the dollars to make the providers ROI worth while.
This is not to say that some providers here don't have really good setups, they do. However since cloud is opaque and you don't really know what your back end infrastructure is you really need to go with a provider you can trust.Last edited by RossH; 07-11-2011 at 08:21 PM.
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07-11-2011, 09:47 PM #28Aspiring Evangelist
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Thanks Ross!
Fail-over (when a server fails) and many other SAN features are available with local storage when you use multiple servers' local storage in a RAID-over-the-network configuration. This is what is generally called a distributed storage network, or a de-centralized SAN. To accomplish this, cost goes up pretty dramatically compared to a single server's local storage, at least for high performance solutions.
Distributed storage has the disadvantage that you must add servers when you run out of disk space. This is a drawback when you need more storage, but not more CPU or memory, and end up having to buy more servers.
The advantage is that additional CPU, Memory, and caching is added to the cluster with each server added.
It just depends on how you look at it.
Yep, it's all based on risk. As the risk goes up (higher loss in revenue, and in some cases even higher loss in profit), the more sensitive the customer (and the provider) gets. Is it worth the risk going with a lesser-known, less-tested solution to save a few bucks (and possibly not sleep at night)? Typically no, but the revenue has to be there to cover the cost.
It's the mutual understanding of the risk and cost from both customer and provider that would make sales a lot easier.
It's always best for a prospect to demand details from a provider regarding what their workloads will run on, and where. I know I would want to know! We've had many clients do just that, visiting the facility to be sure we are who we say we are. I think it's a great idea, even if it takes time and money to do it. It isn't always practical for small customers, though, which is where trust and reviews becomes the determining factor.
EricGenesis Hosting Solutions, LLC (genesishosting.com)
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07-12-2011, 12:24 AM #29Away
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Eric,
Not that I don't believe what you are saying but can you list some other provider that user DSNs (distributed storage networks) as you described or hypervisors that support it truly (Are you talking iSCSI, I'm confused)? I really haven't heard of "high performance" solutions that didn't involve SAN, caching cards and some SSDs but I'm sure I haven't heard of all the technology available today. How does DSN work with no central repository of storage? I must not be understanding the technology.
Agreed.
Agreed.
I appreciate the discussion we are having.Last edited by RossH; 07-12-2011 at 12:37 AM.
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07-12-2011, 01:38 AM #30Aspiring Evangelist
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CA/3Tera's AppLogic uses a distributed storage architecture using local storage (from what I understand... I admit I've never used it, only seen demos). Other products include SeaNodes and LeftHand Networks (now owned by HP).
Most products are virtual SANs as opposed to distributed storage. This includes StorMagic, PHD Virtual's free virtual SAN software. I'm sure there are others, but these don't necessarily scale out when storage nodes are added. When I referred to high-performance, I really meant the difference between distributed storage and a virtual SAN.
It's probably best to look at some of these products' sites to learn about their implementations.
iSCSI is an "interface", not storage architecture. Servers (more specifically HBAs) communicate with storage systems using storage interfaces, typically SCSI or ATA (Serial or Parallel) and when accessed across a network, use a transport such as iSCSI or fiber channel. iSCSI provides a means to transport SCSI commands over an IP network. Fiber channel provides a means to transport SCSI commands over a fiber channel network. AoE provides a means to transport ATA over Ethernet. And FCoE provides a means to transport fiber channel over an Ethernet network. Crazy, yes.
Distributed storage, virtual SANs, and physical SANs can act as "targets" of storage using interfaces such as iSCSI, fiber channel, and even SAS in some cases nowadays. Servers or virtual machines are "initiators" of I/O requests to targets.
Every node is aware of the storage topology and thus can respond to I/O requests and proxy requests to other nodes if necessary, such as if the block being requested/written is on a different node. It's very similar to a pseudo-active/active SAN where only one controller "owns" a LUN and thus processes I/O, RAID functions, and cache sync. If a command is sent to the non-owning controller, the command is proxied to the controller that owns the LUN.
It's typically better to have integrated support in the OS/hypervisor for a distributed storage system so more intelligent decisions can be made for where I/O is processed. In SANs where LUNs are owned by a controller, there are now commands that can be sent back to the server/hypervisor to tell it which network path is best to get to the controller that owns the LUN. Proxying is expensive computationally and latency-wise, so it's best to avoid it if possible.
As a general storage platform where servers access a distributed storage network by simply load-balancing I/O requests among nodes isn't necessarily the best approach for the optimum performance. This can be done on active/active fiber channel SANs. We have done some testing of this configuration and found it can actually hinder performance.
Boy is that true. Definitely grill the provider for information and ask for demos, trials, etc. if you feel uncomfortable. If you feel nauseous, run away, run away.
EricGenesis Hosting Solutions, LLC (genesishosting.com)
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