VCP – VMFS Block Sizes

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A quick post on VMFS block sizes.  There are two configurable VMFS versions in vSphere 5.  VMFS-3 and VMFS-5.  VMFS-3 data stores could be configured to use different block sizes.  The reason for doing this was that based on the block size, a user could increase the maximum size of a single virtual disk.  So the block size dictates the maximum file size for a particular data store.  For instance…

Block size Max size of
1 MB 256 GB
2 MB 512 GB
4 MB 1 TB
8 MB 2 TB

So when you create a VMFS-3 data store, you get the option of selecting the block size…

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Once configured, the block size can not be changed.  If you want to change the block size, you have to migrate data off, blow away the VMFS data store, recreate it, and migrate data back.

This is not the case with the newest version of VMFS, VMFS-5.  Only hosts running vSphere 5 or later support VMFS-5.  Hosts running earlier version wont even be able to see the data stores formatted with VMFS-5.  VMFS-5 has the following improvements.

-VMFS-5 uses a unified block size of 1MB.  The block size isn’t even a configurable options when you add storage as VMFS5. Despite the smaller block size the max file size in VMFS-5 is 2 TB.

-VMFS-5 data stores can be as large as 64 TB.  This is the max size for data stores with or without extents. 

-Physical RDMs in VMFS-5 can be larger than 2 TB in size. 

So there are some significant enhancements in VMFS-5.  And the bonus is that you can upgrade VMFS-3 to VMFS-5 on the fly.  The only gotcha is that the block size can’t be changed.  For instance, let’s upgrade one of my VMFS-3 data stores to VMFS-5.  So here is what we have to start with…

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As you can see, it’s a VMFS-3 file system set with a block size of 8.  Now, click the link up top that says ‘Upgrade to VMFS-5…’.  You’ll get this warning…

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Click OK to start the upgrade…

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As you can see, once it completed I’m now running VMFS-5.  Also note that although VMFS-5 only uses a block size of 1 MB, the block size is still 8MB.   To get the best performance out of the data store, I’d likely want to rebuild it as a native VMFS-5 data store to get the same performance as newly built VMFS-5 data stores.

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