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 File Server VM via RDMs? 
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Joined: Mon Jul 26, 2010 10:58 am
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Post File Server VM via RDMs?
I'm considering moving from NAS stored VM's to locally stored VMs with a virtual file server running on my ESXi4. I think I'm going to run either EON storage (openSolaris Based, ZFS) or FreeNAS (ZFS based). I'm not real smart on file systems, but I believe using local storage to gain the benefits of ZFS would require Raw Device Mapping, right?

As for disks, I'm currently using 4 1TB disks in the openSolaris box. I would buy 4 new drives to transfer everything over to. Which drives should I look at for this from a performance aspect? Hypothetically speaking, what kind of performance should I expect from a box using 7200RPM SATA drives and gigabit eithernet NIC(s)?

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Sat May 28, 2011 9:40 am
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Joined: Mon Mar 16, 2009 10:13 pm
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Post Re: File Server VM via RDMs?
You shouldn't have any issues with ZFS if you use virtual disks instead of RDMs. RDMs are advantageous if you need to import existing disks, need portability or want to use all disk space. Virtual disks have some benefits such snapshots.

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Dave Mishchenko
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Sun May 29, 2011 10:15 pm
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Joined: Tue Nov 17, 2009 6:10 am
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Post Re: File Server VM via RDMs?
Dave.Mishchenko wrote:
You shouldn't have any issues with ZFS if you use virtual disks instead of RDMs. RDMs are advantageous if you need to import existing disks, need portability or want to use all disk space. Virtual disks have some benefits such snapshots.

Hi Dave,

I disagree! In my tests the main trouble are the lack of support for errors in RDMs. If you physically disconnect a disk or the disk generates an error, the virtual controller NEVER pass the error to the guest OS. As a consequence your ZFS pool enters in a infine-loop of waits!

I hope that VMware will fix this behaviour in ESX5 and enable FULL PASSTHOUGH of RDMs to the guests (like do "scsi0.returnBusyOnNoConnectStatus" in ESX 3.5; or ESX 4.x with Windows only!).

Regards.


Mon May 30, 2011 12:05 am
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Post Re: File Server VM via RDMs?
Are you using virtual or physical RDMs?

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Dave Mishchenko
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Mon May 30, 2011 12:26 am
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Joined: Mon Jul 26, 2010 10:58 am
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Post Re: File Server VM via RDMs?
Dave.Mishchenko wrote:
Are you using virtual or physical RDMs?


I assume this was directed at danisoto, as I'm not using any now.

I'm by no means a storage expert, but as I understand VMWare and ZFS, it seems that the benefits of ZFS aren't there if there is an underlying file system residing on the disks. If I implemented a RDM, then ZFS could find/prevent bit rot where if it was a virtual disk, then if I understand correctly it could only prevent corrupted data on the ZFS layer (as opposed to the virtual file system).

Just trying to do the math before migrating data over to a virtual NAS.

What about performance?

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Mon May 30, 2011 4:54 pm
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Post Re: File Server VM via RDMs?
The comment was for danisoto.

Performance wise if you need the best performance you'll want to go with a controller with battery backed write cache. Typically when you're using SATA controllers you don't see the same performance as you would with the guest running natively on the hardware.

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Dave Mishchenko
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Tue May 31, 2011 12:21 am
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Joined: Tue Nov 17, 2009 6:10 am
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Post Re: File Server VM via RDMs?
Dave.Mishchenko wrote:
Are you using virtual or physical RDMs?

Hi Dave,

Tested with both!
The two configurations "filters" connection errors, and translates it to timeouts in the guest.

Please, do this simple test:

Make a RDM (virtual or physicial), connect it to a virtual machine, start the virtual machine... disconnect the physical disk, and see the status of the disk/controller in the virtual machine.

Regards!


Mon Jun 06, 2011 12:03 am
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