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Datastore size for just an OS LUN?

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This seems like a silly question to me, but the alarm is throwing me off.

If I want to create a Datastore that is just a LUN for one OS only, how big should it be?

For instance, in building a Windows Server 2008 R2 machine, I created a LUN on the SAN for just the OS, and set it to 50GB. Then, in vCenter, I add the storage to the ESXi hosts as a datastore (lets call it OSLUN). Next, I create a VM and use OSLUN as it's hard disk, provisioning 40GB (thinking that's all I'll need). Now, becuase I'm choosing 8GB of RAM, the total of the VMDK ends up being 48GB. vCenter throws up a "Datastore usage on disk" alert saying out of 49.75GB there is only 1.24GB left.

That's all logical to me, except the alarm makes me think that I'm somehow going to run out of space for the datastore...should I just ignore this alarm? I don't see how I'd go over the allocated space for the OS LUN. πŸ˜•

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Immortal
Immortal

Unless you were to store those things on different sets of spindles it won't make too much difference. Keep your environment as simple for as long as you can. Add complexity when it solves a specific problem not just because you can.

-- David -- VMware Communities Moderator

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Hi Themind,

If you never use the Snapshot technology, you can ignore the alert. The few logs should not consume Gigs.

But as soon as you will backup your VM with a VMware aware solution, or use any other solution that automatically creates a snapshot, your LUN will be full and prevent your VM from running anymore...

Regards

Franck

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Contributor

Ok, then if I choose to use the snapshot technology, what should the proper store/LUN size be for this specific instance?

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Leadership
Leadership

It will depend on how long you will keep the snapshot and the amount of new data generated during this period. Not easy to guess...

You always have to keep about 10% of free space in your LUN. Add an additional 10% margin, in case... So for your single VM with 8Gb memory, a 55~60 Gb LUN would be safer.

If you have more VMs in a single LUN, this free space is shared as you can assume that not all VMs will have a big snapshot file at the same time.

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I hadn't even thought that I'd have to plan in space for a snapshot for the backup software. Is there no way to point the snapshot to a different location (I'm guessing that since I said store the disk file with the machine on VM creation, that's a no)?

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Immortal
Immortal

Why are you creating a LUN for just one VM? A small LUN can easily get you in trouble. Create a reasonable sized LUN for a datastore to contain multiple virtual machines and their logs, snapshots, swapfiles etc.

-- David -- VMware Communities Moderator
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I knew someone was going to ask this. From what I've read, it's best practice to put the database and logs on separate luns all together for Exchange 2010. I'm doing so, and in doing that, the OS would need to be on it's own LUN.

Edit: Maybe the OS wouldn't have to be...but that's what I figured doing since the others would be. I may just be able to put the OS into the regular large VM LUN...

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Immortal
Immortal

I would have a look at http://www.vmware.com/solutions/business-critical-apps/ before you start creating a bunch of tiny LUNs.

-- David -- VMware Communities Moderator
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Dstavert,

Thanks for the help. I've been there, I have the Exchange "Getting Started" kit. You do realize that even in the demonstration on that Business Critical Apps page, "[8,000 to 16,000 End User Mailboxes|http://www.vmware.com/files/pdf/resources/16000_exchange_on_vmware.pdf]", they also used separate LUNS for the DB and Logs?

I had originally thought that there'd be a performance problem putting the OS VMDK with a normal larger LUN for VMs, but in retrospect, I don't think it would make a difference. I have yet to see anyone state that the Exchange DB and Logs should go there too, however.

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Immortal
Immortal

If you are faced with 8000 mailboxes then my apologies. If you have a more modest install then I wouldn't complicate my life with tiny LUNs. How many drives in your SAN? How fast are the disks?

-- David -- VMware Communities Moderator
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Ha, no I don't have 8,000 mailboxes, I was just pointing out that everywhere seems to suggest separate luns for DBs and Logs, even on as small of implementations as mine, for Exchange 2010. I've got 100 mailboxes, very small, and a P4300 G2 SAS Starter SAN, and maybe it would make sense to just throw it all in one place...I just don't want to have to revert to separate luns later because I didn't do it in the beginning. I get your point of view, though.

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Immortal
Immortal

Unless you were to store those things on different sets of spindles it won't make too much difference. Keep your environment as simple for as long as you can. Add complexity when it solves a specific problem not just because you can.

-- David -- VMware Communities Moderator

View solution in original post

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Contributor
Contributor

I have to agree with that philosophy, thanks for the help!

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Immortal
Immortal

If you are relatively new to ESX(i) I would spend some time going through some of the free online training. http://communities.vmware.com/community/vmtn/vsphere/quickstart It really helps transitioning physical to virtual. Virtual is an opportunity to take a fresh look at how you do things.

-- David -- VMware Communities Moderator
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Immortal
Immortal

Very welcome. Have fun.

-- David -- VMware Communities Moderator
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