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Disk in VM - statement

Posted on 2011-09-19
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"In VM environment, HDD can share only one pool of large IO backplane"

what exactly does this statement mean? is it accurate?

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Question by:25112
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ID: 36564017
Could you provide in what context, you seen this statement?
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by:25112
ID: 36564035
our hardware vendor who maintains and supplies our servers want to go virtual.. we asked for multiple drives for databases (data on one drive and index on one and so forth).. they sent the above statement as an indication that they will provide only one drive for each VM other than C drive, and we need to create sub folders, as it does not make any real difference in a VM environment.. and everything is coming from one big backplane..

(can you please tel what is a backplane)..
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Andrew Hancock (VMware vExpert / EE MVE^2) earned 500 total points
ID: 36564081
Okay, I think I understand what your vendor is stating. When creating datastores for virtual machines, you generally want to use as many fast disks as possible connected to a fast RAID controller striping and using maybe RAID 10. All your virtual machines disks, you could have a virtual disk for your C: and a virtual disk for your D: drive (db), and a virtual disk E: (logs).

But all these disks will be stored on the same set of RAID 10 disks, connected to the same controller, plugged into the backplane on the motherboard (which is where the bottleneck) will be.

Could also be backplane on the chassis that connects the disks.

So what they are stating is true-ish.

Make sure you have a very fast datastore to store all your VM disks, all VM disks are using the same datastore so the IOPs for the datastore will be the same for ALL disks. (as all disks are stored on the same datastore, RAID set, and controller)
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by:25112
ID: 36566576
thanks for the explanation..

so, unlike in the physical world, it is not possible to have each drive with its own spindles, in virtual servers?
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ID: 36566732
Well it is, but it's not efficient, and you will not get the performance benefits. You need to start thinking virtual, and not physical.
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by:25112
ID: 36567611
OK: so you are saying having one disk for all VM servers is the way to go from now on?

what would be the "performance benefits" that we would gain in having the one big disk

are there exceptions? like busy database where as an example, Log is written sequential and data files in random etc.
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ID: 36567657
No exceptions. You need to create the fatest datastore, RAID array as technically possible to store ALL your VM disks.

Create one large RAID 10 Array containing ALL your disks, the performance is N x disks. Each disks IOPs is combined!

More disks = more performance = more IOPS for your virtual machine disk.
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by:25112
ID: 36567848
>>Create one large RAID 10 Array containing ALL your disks, the performance is N x disks.

so 200 disks under one controller is better than 50 disks under one controller - is that what you are saying?

is that the same as
"Multiple heavily used virtual machines concurrently accessing the same VMFS volume or multiple VMFS volumes backed by the same LUNs can caused decreased storage performance." www.vmware.com/files/pdf/sql_server_virt_bp.pdf
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Yes, that's exactly what I'm stating, but those values are extreme.

e.g. 16 disks are better than 8 disks.

But in large SANs, you would usually stripe all the disks together off one controller.

That statement is also true about LUNs, so you would balance LUNs on a SAN, e.g. you would not put all your heavy VMs on the same LUN, you would create another LUN. LUN access is different because of FC or iSCSI connections, and less bandwidth
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by:25112
ID: 36579918
hanccocka, thanks for your input..

please confirm i am understanding it right:
a controller can have 16 disks. then this could be carved as 4 LUNs as an example (4 LUNs will have their own IO and spindles distinct of each other). Then each LUN may server x number of VMs?
and this would true of the original statement of the vendor, that I mentioned?
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ID: 36579944
Note quite. A Controller can create a single Array of 16 disks for example, this can be split into individual units (sometimes called Virtual Disks, Logical Disks or LUNs, different vendors use different names).

BUT, the LUNs that are carved out from the Array, exist somewhere on the 16 disks.

So those 4 LUNs are part of the Array (16 disks - those LUNs are striped across the 16 disks).

You could however, define 4 Different Arrays, and 4 Different LUNs, so there is a one to one relationship.

Each LUN is then stored on 4 Disks. Each LUN has it's own IO, and spindles distinct from each other.

But in configuring it that way, you will only get the IO, of the combined 4 disks for the LUN. (rather than  16)
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by:25112
ID: 36580402
>>You could however, define 4 Different Arrays, and 4 Different LUNs, so there is a one to one relationship.... But in configuring it that way, you will only get the IO, of the combined 4 disks for the LUN. (rather than  16)

if we define only 1 array for 4 LUNs, then there is 1:4 mapping.. does that give IO of 16 disks (rather than 4)
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Yes, because the 4 LUNs are stripped across all 16 disks. (because the Array is defined across 16 disks).
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by:25112
ID: 36580580
OK- good;

is this concept unique for the virtual world, or shared with the physical SAN, also?

(one big datastore has many disks and LUNs carved from it)
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LVL 120
ID: 36580623
It's the same for SAN as well.

some benchmarks for you so you can see more disks is better...

2 disks - 84.5 MB/s Write,  150 MB/s Read (SATA 5,400rpm)
4 disks - 196 MB/s Write, 276 MB/s Read ((SATA 5,400rpm)
8 disks - 212 MB/s Write,  287 MB/s Read (SATA 5,400rpm)

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by:25112
ID: 36581215
that a good statistic.. thanks..

just wanted to quickly revisit the 'no exceptions' topic we discussed...

they (MS MVP) mention separate RDM for each database file. (multiple arrays) is optimal for busy databaes.. so that means the benefit you should mentioned is forfeited.. but they are saying there is a benefit "eliminates hypervisor and less CPU and faster IO"

is this true for all VM products..
how do you balance the two... if you go one way there is one benefit.. the other way another benefit.. i would appreciate your thoughts on that...
People get in trouble trying to virtualize SQL Server by not treating it as a SQL Server and planning things like Disk IO for SQL.  If you think you can present a single LUN from a SAN to a host, format it for VMFS and then carve up 16 servers worth of VMDK's off that LUN and have one or more of those be for SQL Server and you are going to have good performance, your up for severe disappointment.  You still need to have dedicated disk arrays from the SAN for database files, log files, and tempdb, and they need to be presented to the host as separate LUNS, and ideally, they will be RDMs and not VMFS volumes, this eliminates the hypervisor from the storage utlization and provides faster IO with lower CPU usage.  The same prinicpals for RAID configuration that you'd apply to a physical server still apply to a virtual machines underlying disk IO subsystem.

http://social.msdn.microsoft.com/Forums/en-US/sqldatabaseengine/thread/aeb4204f-c1d8-4f95-a6e5-3bdf696ad84a/

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LVL 120
ID: 36581279
There is some truth to that statement.

But then recently this was discovered

RDM versus VMDK performance

Conclusion: VMFS and RDM have similar performance. Don’t choose RDM for performance.
Source:http://www.vfrank.org/2011/03/22/performance-rdm-vs-vmfs/

My Advice, you would need to test what is best for your environment, and database.

You can also have RDMs which SPAN multiple Disks on a SAN!

Traditionally large SANs, have many many disks, and the Storage is carved out of a very large storage pool.

Some organisations, use RDM because or performance, but as the link above proves, sometimes they are no faster than VMDKs (VMware Virtual Disks)
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by:25112
ID: 36582130
helpful knowledge thx
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no problems
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