Alrighty then, it's time to integrate your shiny new HP D2D StoreOnce appliance into your Data Protector environment. You're an old hand at DP, so that's no issue. For the new D2D, you've read the Concepts Guide, Install & Config Guide, User Guide, and finally the all-important HP D2D Backup Systems Best practices for VTL, NAS and Replication implementations document. Among all the data swimming in your head, you remember reading on page 25 of that last tome ...
Use a block size of 256KB or greater. For HP Data Protector and EMC Networker software a block size of 512 KB has been found to provide the best deduplication ratio and performance balance.
Not a problem. You know the default for LTO/Ultrium is 64k, and you've typically increased that to 256k which has historically been the sweet spot for backup to physical tape. So now you're going to set your VTL tape device block size to 512k. What could possibly go wrong?
Well, a couple of things, specifically. If your Media Agent (MA) host is running MS-Windows or HP-UX, you may encounter a message similar to the following.
Media Agent host is HP-UX
If you are running HP-UX 11i v2 (11.23) or earlier, the stape driver is limited to a maximum block size of 256k. Attempting the use of a larger block size until recently would work for the DP write operation but fail miserably on restores. This was caused by the stape driver quietly "chunking" larger block writes down to multiple 256k block writes. Details are included in Document ID KM915678 and Document ID c02256887.
Don't fret if you're stuck at 11i v2 which dictates a max block size of 256k. It's infinitely better than the default of 64k where you will see diminished D2D performance and virtually no deduplication. Sure, 512k may be a tad better, but 256k is still good.
For those running 11i v3 (11.31), it's somewhat of a mixed bag. To guard against unpleasant surprises that may result from the quiet "chunking" noted above, the latest MA patch for DP has imposed an artificial block size limit of 256k for HP-UX. That's good except that the stape driver in 11.31 can in fact handle larger block sizes like 512k. Your options then if your media host is HP-UX 11i v3 are:
- Stay at a 256k block size with tape devices (D2D and physical).
- Remove the latest MA patch (PHSS_41963) and revert to the next-to-latest (PHSS_41460).
- Keep the latest MA patch, open a support call, and request the SSP for QCCR2A31441.
Options 2 and 3 will allow you to successfully use a 512k device block size with DP and HP-UX 11i v3. From reading the lab case notes, it looks like the SSP provides updated BMA and MMA binaries that will not impose the 256k limit on HP-UX 11.31 where such restriction is not required.
Media Agent host is MS-WIndows
Here's where things may get a little cloudy. If you experience an error in DP attempting to use a tape device configured with a 512k block size, the cause is most likely a configurable parameter called MaximumSGList which is used by the service for your Fiber Channel (FC) Host Bus Adapter (HBA). A little sleuthing is required to track down the proper service to modify in the registry. A quick trip to Device Manager will give us a start. Expand SCSI and RAID controllers then locate the FC controller(s). In my example, we have a pair of QLogic HBAs.
Check the properties of the HBA and look specifically for the Service property on the Details tab. Here we see a value of ql2300.
That tells me I need to dive into the registry at \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servi
Note that there is no current key named MaximumSGList. In the absence of this key, the service will be operating at a default Scatter/Gather value. On this particular host, I happen to know that the default value will support up to a 256k block size. If we wanted to allow operation at 512k, it would be necessary to add a new DWORD "MaximumSGList" with a decimal value of 129. The formula is (BlockSize/4096) + 1.
Mention is also made of MaximumSGList in DP's help facility. For a direct hit, search on example of changing block size on windows media agent.
Note that a restart is necessary to implement the new value before testing.
With our addition made and the reboot complete, I reconfigured one of the D2D VTL drives (LTO4 emulation) to use a block size of 512k. The format of a virtual cartridge was successful at 512k and so was the subsequent backup of data to the cartridge.
It is possible to achieve a 512k block size with MS-Windows and HP-UX 11i v3. I haven't addressed Linux as a Media Agent host primarily because it wasn't possible in the test environment to which I am so graciously allowed access. It's also important to note that you shouldn't go crazy with MaximumSGList and arbitrarily set it to some really high value. As noted in Microsoft Support Article ID 280793:
MaximumSGList is a value between 0 and 255. Use this adjustment sparingly as scatter/gather lists consume nonpaged memory.
My profound gratitude goes to a number of people who responded to my internal queries: Christian, Paul, Andreas, Jaime, Joerg, Mike, Nikhil, Harold, and anyone I might have missed. It is indeed a pleasure to work with such knowledgeable and helpful folks.
Have we missed anything? Do you have experience that can augment or clarify what is documented here? If so, please share with us using the comments section so that we may all benefit!