Actifio CDS 2TB MDisk limits
**** updated 7 Aug 2016 - reformatted and corrected out of date info ***
Background
Depending on your Actifio CDS Platform version, certain disk controllers may not be able to present 2 TB MDisks to Actifio CDS.
Determining 2 TB MDisk support
When looking at the IBM Support Matrix here (Depending on which CDS Platform version you are on):
IBM SVC 7.5 (brought in with CDS 7.5.0.1)
IBM SVC 7.3.0.8
IBM SVC 6.3.0.3
You will spot a column in storage controller support that talks about support for MDisks larger than 2 TB.
You can use the column to determine whether your storage controller can present a 2 TB MDisk to CDS.
Background on how support is determined
When IBM brought out SVC in 2003, it was totally 32 bit, meaning no volume or disk could be larger than 2 TB in size.
Having 32 bit limitations was not an issue as there were few disk subsystems at the time that supported LUNs greater than 2 TB (the DS6000 for instance cannot create a volume larger than 2TB, so the SVC limit shown above is actually a DS6000 limit).
However as time moved on, the SVC dragged its heels and eventually moved with the times (as did IBMs other products).
I wrote about this issue here:
Breaking through the 2TB barrier | Aussie Storage Blog
Anyway... when doing testing for greater than 2TB MDisks, the SVC team required that every disk controller be willing to report media errors for any LBA that it presents to SVC. After all, if the storage cannot report media errors, then the SVC will not be aware that its underlying storage has unreadable blocks. It seems some manufacturers did not update their 32 bit media error reporting when they moved to 64 bit.
For external array manufacturers I believe IBM simply ask the manufacturer to confirm in writing that their storage can report media errors for every LBA. For their internal products (like DS8000 and XIV) they do the same with the relevant development teams. (I cannot imagine they could easily test this as generating synthetic media errors would require monkeying around in the firmware of an external array - or hitting it with a hammer)
Background on XIV Support in SVC 6.3
The limit below no longer applies in CDS 7.3.0.8, I leave the info here for historical purposes only.
One curiosity is XIV. We know XIV can support huge volumes, but the SVC matrix does not support them. Why?
The XIV development team told the SVC team that the XIV cannot report media errors for LBAs higher than the 2 TB mark.
The technical reason for this was never explained to me but I think XIV felt that since they always stored two copies of each 1 MB partition and used some metadata and LRC checking to ensure they always presented a good copy, what was the point of reporting something they felt was not possible. The SVC guys were slightly stunned and did not agree.
A cold war ensued with the XIV team refusing to change their code and the SVC team refusing to drop their stance.
End result... still no support.
The curious thing is this.... in SVC code version 6.2 when I tested greater than 2TB support... where I presented a 3 TB XIV LUN to SVC... the SVC would only 'see' a 2 TB LUN... in other words it ignored the bonus LBAs. However in 6.3 code this seems to have been broken and I was able to present an XIV LUN of any size and the SVC could see it and use it. When I reported this to the SVC team, they were surprised but did not appear to do anything about it.
Their stated direction was to use the vendor reported info to determine if the MDisk controller was on the 'trusted list'. Meaning that the SVC would only report an MDisk size greater than 2 TB if the controller was on that trusted list. Since XIV is not... it should not work.
Maybe the list they are using has XIV on it by mistake, or the list checking method was broken in the revision of 6.3 I tested.
Either way... the support matrix says no.... and thats what we have to live by.
It's also useful for all other maximum configuration limits.
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