I have previously discussed one of the big problems with the NetApp NearStore VTL here and here: it uses a type of RAID that is more dangerous than RAID-6 (including RAID-DP), and is, in fact, more dangerous than RAID-5.
Now I was content to describe the NearStore VTL as merely "more dangerous" than EMC's EDL at the time. After discussing the issues with Alex McDonald on The Missing Shade of Blue, I think I came to the conclusion that:
- The NearStore VTL is, basically RAID-5. Yes, it is called VTL RAID by NetApp marketing, and yes there are some differences, but fundamentally, the risk and rebuild times are equivalent to RAID-5 (unless you aren't actually using the disk volume!).
- In some respects, VTL RAID is worse than RAID-5 in terms of risk of data loss, as it tends to disperse some virtual cartridges across more than one RAID group. Meaning that the failure of a RAID group can impact a greater number of virtual tape cartridges than its capacity might indicate.
All this lead me to characterize VTL RAID as having 135% to 270% more risk of catastrophic data loss due to double disk failure than a RAID-5 volume of the same size. And all I was further able to conclude about the risk of a NearStore VTL vs. an actual EDL was that that the risk of a NearStore is much greater than an actual EDL that uses RAID-6.
It appears now however that we can be very specific about how much "much greater" is when it comes to the risk of a NearStore vs. an EDL.
According to the latest data cited by Alex: "Over a period of 5 years, RAID-5 is 4,000 times more likely to suffer from data loss than RAID-DP." His emphasis. Quote source is here.
And that is not accounting for the further risk VTL RAID introduces due to self tuning.
Now none of this has really been debated by NetApp. Based on comments by Alex, I think that he and NetApp fundamentally agree with the reasoning.
Which means there are really only one possible conclusion that we can draw: the NearStore VTL is 4,000 times or more likely to suffer catastrophic data loss than the EMC EDL.
And here is the real, fundamental outcome of this line of reasoning--we have two options:
- NetApp probably shouldn't be selling, and customers probably shouldn't be buying, a NearStore VTL because the risk of catastrophic data loss is just too high. If it really does put your data at 4,000 times greater risk than competitive products, why even bother considering it?
- RAID-6 and RAID-DP is just not that important. The only way to avoid option 1 above, is to say that RAID-5 doesn't really pose a risk in the first place: i.e. 4,000 times nothing is nothing. That doesn't seem remotely consistent with what NetApp has been saying for the past few years, or what Alex argues in any of his columns, but it is really the only way you can conclude that a NearStore VTL has an acceptable level of risk to it.
Hrmm. Interesting stuff.