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Why align a virtual lun

Posted on 2010-09-21
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Hi,

The need of aligning a raw lun or ISCSI lun  I understand.
But why align a virtual lun, it is just a file.

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Question by:MTasova
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by:madan1278
madan1278 earned 400 total points
ID: 33723672
If the virtual lun resides on a shared storage, the VM can be failed over seamlessly via the vmmotion network.
In a virtual lun acts like a physical to the actual VM's.
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andyalder earned 800 total points
ID: 33723728
The virtual KUN may be just a file but files are made up of blocks and clusters and these should be aligned to an even boundary to avoid split I/Os. See the drawing on page 5 of http://www.vmware.com/pdf/esx3_partition_align.pdf. (it can get even worse if the SAN LUN itself is a file under iSCSI target software, then there's yet another layer where mis-alignment can occur)
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by:BloodRed
BloodRed earned 800 total points
ID: 33724515
Essentially, it is still a file which is accessed by looking up the associated blocks on the underlying disk subsystem.  If the file is alligned, one virtual block will (well, should) map to one physical block and one I/O operation is used to read it.  If the virtual blocks are mis-aligned and span multiple physical blocks, each 'read' of a virtual block will require the disk head to read multiple physical blocks.  In other words, there will be more read/write operations required to write a single virtual block of data should the file be mis-aligned.
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