Xenserver and Storage
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So you have a VM on each host, and you give it all the local storage. It then allows you to connect the host to it via some protocol (iSCSI, NAS etc)? Or does the VM has some sort of extra hook into the OS to manage and share the storage?
Does it basically just keep the storage volumes on each host synced and identical?
What kind of overhead does this create (ie if I have 6Tb in each server, does that mean I actually only have 3Tb of usable space since I need 2 copies of everything, 1 for each server)?
Is there a need for a dedicated link between hosts for sync traffic?
Starwind's stuff is free, which is cool. Is the paid version particularly expensive? I am thinking support would be a good idea, if only for a year.
This just sounds too easy and/or good to be true. As it sounds like I just need to add drives to my 2 hosts and setup some free software and I'd be set. So I am just making sure I know about as many of the considerations as possible before I run this up the flag pole for a budget.
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@jrc said in Xenserver and Storage:
So you have a VM on each host, and you give it all the local storage. It then allows you to connect the host to it via some protocol (iSCSI, NAS etc)?
It is VSAN if it uses iSCSI. It is VNAS if it uses NFS or SMB.
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@jrc said in Xenserver and Storage:
Or does the VM has some sort of extra hook into the OS to manage and share the storage?
That would not be VSAN then. It's really SAN. Not something randomly being called SAN. It's just a SAN that isn't on its own hardware.
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@jrc said in Xenserver and Storage:
Is there a need for a dedicated link between hosts for sync traffic?
Yes, just like with normal SAN.
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@jrc said in Xenserver and Storage:
Starwind's stuff is free, which is cool. Is the paid version particularly expensive? I am thinking support would be a good idea, if only for a year.
Not too bad. Way less than something like VMware's VSAN.
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@scottalanmiller said in Xenserver and Storage:
@jrc said in Xenserver and Storage:
Or does the VM has some sort of extra hook into the OS to manage and share the storage?
That would not be VSAN then. It's really SAN. Not something randomly being called SAN. It's just a SAN that isn't on its own hardware.
So the appliance then makes use of the virtual hard drives you assign to it for the storage your host then uses? How do you get past the 2Tb limit in this then??
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@jrc said in Xenserver and Storage:
This just sounds too easy and/or good to be true. As it sounds like I just need to add drives to my 2 hosts and setup some free software and I'd be set. So I am just making sure I know about as many of the considerations as possible before I run this up the flag pole for a budget.
Don't think of it that way. This is exactly what we've been preaching for forever. When we say "no one needs a SAN", this stuff is why and long has been. This is just one of the ways to have RLS... you can see when I was writing about RLS:
http://www.smbitjournal.com/2013/07/replicated-local-storage/
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@jrc said in Xenserver and Storage:
@scottalanmiller said in Xenserver and Storage:
@jrc said in Xenserver and Storage:
Or does the VM has some sort of extra hook into the OS to manage and share the storage?
That would not be VSAN then. It's really SAN. Not something randomly being called SAN. It's just a SAN that isn't on its own hardware.
So the appliance then makes use of the virtual hard drives you assign to it for the storage your host then uses? How do you get past the 2Tb limit in this then??
You don't. Anything on top of Xen is going to have that limit.
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@scottalanmiller said in Xenserver and Storage:
@jrc said in Xenserver and Storage:
@scottalanmiller said in Xenserver and Storage:
@jrc said in Xenserver and Storage:
Or does the VM has some sort of extra hook into the OS to manage and share the storage?
That would not be VSAN then. It's really SAN. Not something randomly being called SAN. It's just a SAN that isn't on its own hardware.
So the appliance then makes use of the virtual hard drives you assign to it for the storage your host then uses? How do you get past the 2Tb limit in this then??
You don't. Anything on top of Xen is going to have that limit.
Then how on earth does that solution scale like they say it does? That means you have a limit of ~32Tb of attached storage (Xen's 16* attached VHD limit and 2Tb per VHD limit). How does the virtual appliance handle getting beyond that?
*I could be remembering the number of attached HDD limit wrong, but I do recall there is one and it is low, but I ran into with Unitrends backups more than once.
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@jrc said in Xenserver and Storage:
What kind of overhead does this create (ie if I have 6Tb in each server, does that mean I actually only have 3Tb of usable space since I need 2 copies of everything, 1 for each server)?
It's network RAID 1 in a two node case. You lose 50% of capacity. But you would with a normal SAN, too. Everything is the same as a normal SAN. So if you have 6TB on each of two nodes, you get 6TB usable by the cluster.
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@jrc said in Xenserver and Storage:
Then how on earth does that solution scale like they say it does?
Starwind can use as many 2TB chunks as you want on any given node. And it can use as many nodes as Xen supports. Starwind will use everything Xen can throw at it.
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@jrc said in Xenserver and Storage:
Then how on earth does that solution scale like they say it does? That means you have a limit of ~32Tb of attached storage (Xen's 16* attached VHD limit and 2Tb per VHD limit). How does the virtual appliance handle getting beyond that?
You have to work around the 2TB limit in another way, but Starwind will use it regardless of how you get it there. So Starwind definitely does not have that limit. But trying to use Xen's 2TB limit system under it will create limits on the Xen side.
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Remember, we say Xen but these limits are 100% XenServer. If you were on Xen, you'd have none of these limits.
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Would spanning the drives within the guest address this issue as far as the guest cares? At 2TB we're discussing file shares and large databases anyways.
Things that simply need storage, continuous or not when "physically" looking at them. Within the guest OS simply address the limit there. . .
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@scottalanmiller said in Xenserver and Storage:
@jrc said in Xenserver and Storage:
Then how on earth does that solution scale like they say it does? That means you have a limit of ~32Tb of attached storage (Xen's 16* attached VHD limit and 2Tb per VHD limit). How does the virtual appliance handle getting beyond that?
You have to work around the 2TB limit in another way, but Starwind will use it regardless of how you get it there. So Starwind definitely does not have that limit. But trying to use Xen's 2TB limit system under it will create limits on the Xen side.
I'm sorry of this sounds dense, but what??
(And yeah, I realize when We/I say Xen we mean Xenserver and not pure Xen).
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@dustinb3403 said in Xenserver and Storage:
Would spanning the drives within the guest address this issue as far as the guest cares? At 2TB we're discussing file shares and large databases anyways.
Things that simply need storage, continuous or not when "physically" looking at them. Within the guest OS simply address the limit there. . .
That's true. But only to the total limit that XenServer can provide.
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@scottalanmiller said in Xenserver and Storage:
@dustinb3403 said in Xenserver and Storage:
Would spanning the drives within the guest address this issue as far as the guest cares? At 2TB we're discussing file shares and large databases anyways.
Things that simply need storage, continuous or not when "physically" looking at them. Within the guest OS simply address the limit there. . .
That's true. But only to the total limit that XenServer can provide.
Which the limits are listed here.
255 virtual drives per VM (assuming the guest OS can support it). So at the most for a single VM you could have 521,220 TB's worth of space. . .
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@jrc said in Xenserver and Storage:
@scottalanmiller said in Xenserver and Storage:
@jrc said in Xenserver and Storage:
Then how on earth does that solution scale like they say it does? That means you have a limit of ~32Tb of attached storage (Xen's 16* attached VHD limit and 2Tb per VHD limit). How does the virtual appliance handle getting beyond that?
You have to work around the 2TB limit in another way, but Starwind will use it regardless of how you get it there. So Starwind definitely does not have that limit. But trying to use Xen's 2TB limit system under it will create limits on the Xen side.
I'm sorry of this sounds dense, but what??
(And yeah, I realize when We/I say Xen we mean Xenserver and not pure Xen).
Starwind isn't a bottleneck here. If your goal is to scale to huge sizes, Starwind will do it. But Starwind, as a guest on top of Xen, requires you to provide storage to it to use. If you can't provide the storage, Starwind will just wait on you. You asked how Starwind can scale as large as they claim - but that's not what you are really asking about. Starwind is in no way failing to scale, it's XenServer failing to give Starwind enough storage.
Use something other than local disks on XenServer, switch to Xen, KVM, Hyper-V or ESXi and Starwind will scale beyond any useful sizes for those systems. XenServer has an artificially enforced limitation here, that exists only to cripple its local storage capacity, that cannot be easily worked around. But don't think that Starwind is failing to scale, that's not the issue.
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Ok, let me take a step back here.
On the Host you install a VSAN controller VM, you then attach storage to this VM which it will then use as it's VSAN storage space and map that over to the host via iSCSI. Am I correct so far?
Allright, so with Xenserver, the max attached VHDs would be 16, at 2Tb each. And since this VSAN VM is to be used to home all your VMs on the host, you'd need it to have a fair bit of space. So having roughly a 32Tb limit could be a problem. Does this mean you'd need to have a second VSAN VM in that host, therefore upping that limit to 64Tb? And does the VSAN OS handle spanning the data across all 16 VHDs?
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Switching to pure Xen, you lose XAPI which is what makes XenServer useful. Without XAPI tools like Xen Orchestra simply don't exist for the space. At least not to the same extent.
