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VMWare : Datastore on iScsi SAN or on local HD - need advices

Posted on 2010-08-24
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Last Modified: 2013-11-14
Hello Community,

I need an advice about Datastore storage. Situation : ESX Servers (DELL PE 2950), SAN iScsi (DELL MD3000i), company between 70-80 people.

What are the pros and cons of placing the datastore of a particular VM (for example a SQLServer 2005, or a File Server, or a TS Server) on the ESX server HD, and not on the MD3000i ?

Could the performance increase / decrease, assuming that the internal disks are "good". I'm for example thinking of migrating a File Server as a VM (1 To of data, about 1M files), and want to choose the best solution for the placement of the datastore.

Thanks in advance.
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Question by:fcomte
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by:bgoering
bgoering earned 150 total points
ID: 33510396
Disk performance will be pretty similar whether the data is on local storage or on a SAN device provided all of the following are true when comparing one option to another.

1. Number of disks are the same
2. Raid configuration (RAID 5, RAID 10) is same
3. Disk types are the same (SAS, SATA)
4. Caching capabilities are the same

You kind of have to watch that last one on local storage, some of lower end raid controllers don't have batter backed write cache, wheras your MD3000i will.

Now a definate advantage to using a SAN device comes in when you add more ESX servers and vCenter. VMs must be on shared storage to take advantage of features such as vMotion, DRS, and HA.

Hope this helps
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by:paulsolov
paulsolov earned 100 total points
ID: 33510418
PROs of local storage

1. May be faster depending on RAID, # of Drives, and type of drives
2. Less administraton

PROs of SAN

1.  Shared storage, allows you to connect multiple ESX host for growth
2.  Separate subnet for storage
3.  More storage capability then local storage
4.  Easier to grow LUNs vs local storage

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Joshua1909 earned 250 total points
ID: 33510448
There are a few things to take into consideration:
- Disk Redundancy: Are the internal disks and SAN both configured with RAID?
- Disk I/O: iSCSI is generally over 1GB ethernet, even if the ethernet is dedicated to the VM traffic, it will most likely be slower than local storage (eg. SAS operates at 3 and 6 Gbps) For that reason I would be highly reluctant to put something like SQL on an iSCSI SAN.
- Server Redundancy: If you have centralized storage and you have vSphere or VirtualCenter you can use VMware's HA and DRS to make your servers redundant...for example you can migrate VM's between the physical server's because they share the storage. If you don't have vSphere or VirtualCenter it's still possible, but it's a very manual process.
- Speed: This is the biggest factor for me. I have a couple of iSCSI SAN hosting VMs, and when the workload is low/medium for my network the performance is fine, but compared to a FC SAN or local storage you can really tell the difference when load increases.
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by:Gustav Brock
ID: 33512829
Popping in as I have similar considerations:

Are any of you using truncated two- or four-port NICs for the iSCSI traffic?

/gustav
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LVL 118
ID: 33514139
don't forget if your serious about iSCSI to use iSCSI TOE cards, software iSCSI initiator has it's limits.
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LVL 118
ID: 33514177
It order of performance

1. FC
2. iSCSI
3. NFS

although iSCSI and NFS, I've seen similair performance if using jumbo frames and good networking. Just don't assume because you've got a network, you can use it, it needs to be "tuned" for good iSCSI and NFS performance.

use a dedicated storage LAN for iSCSI and NFS.

Unless you are considering running your iSCSI over 10Gbe ethernet.
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ID: 33514187
obviously it's also based on cost vs performance.
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LVL 118
ID: 33514213
I did some benchmarking many moons ago with 2GB FC, iSCSI (software and hardware) NFS, and local Ultra 320 disks in ESX using IOMeter.

Local Disk and FC were very close, hardware iSCSI, NFS, Software iSCSI, using NetApp filers, which can do FC, iSCSI ands NFS all in the same SAN/NAS.
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by:bgoering
ID: 33514480
@cactusdata - You should open a new thread for your question, while similar it is a bit off topic for this one.
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by:fcomte
ID: 33514493
Hello guys, thanks for your answers, I will compulsate all and give a feedback soon !
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by:Gustav Brock
ID: 33518481
So DELL MD3000i doesn't offer port trunking?

/gustav
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by:Joshua1909
ID: 33518998
@ hanccocka
I'll have to double-check to be sure, but I believe hardware initiators are not currently supported in ESX 4
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by:paulsolov
ID: 33523284
ESX fully supports iSCSI HBAs with TOE, upgrading with one in a system right now.
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by:bgoering
ID: 33524575
Yes - it does, just make sure your HBA is on the compatibility list (http://www.vmware.com/go/hcl). My old QLogic iSCSI HBA (5042C I think) worked fine on 3.5, but is no longer supported on 4.x

Good Luck
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by:Joshua1909
ID: 33526157
Ok I checked my info. It was a few months ago on a VMware course they mentioned they were having issues with IBM iSCSI HBAs.
As the others say, check the HCL
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LVL 118
ID: 33546984
QLogic or Emulex iSCSI cards, (IBM just re-brand Qlogic, as do most other Vendors)
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LVL 118
ID: 33546992
I don't know where you got the impression that hardware initiators are not currently supported in ESX 4!!!

they've been supported since ESX 3.0! (and working in 2.5 but not supported!)
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by:Joshua1909
ID: 33547294
If you look at my comment, you'll see I said it was mentioned on a VMware course. To be specific it was on the VMware Vsphere: What's New course to upgrade from VCP3 to VCP4.
Wasn't in the course material, but the instructor advised they were having trouble with some iSCSI HBAs. And yes I do realise they just re-brand.
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Author Closing Comment

by:fcomte
ID: 33557881
Thanks for your proposals. I'm confort in the idea that virtualization is not the answer to everything. Then in our projects there will certainly be some local datastores for Files servers.
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