I’ve read with interest this week the posts on wide striping and the consequent expansion to thin provisioning. Here are some of the highlights:
First there’s Martin Glasborow’s post, which discusses whether wide striping and thin provisioning should be chargeable items. I’d go a step further than Martin and suggest that thin provisioning (TP) should also be free; after all, over time thin provisioning becomes fat provisioning without some kind of reclaim technology and there’s only value to TP with something like Zero Page Reclaim to get back those unused blocks.
Then there’s Hu Yoshida’s post referring to the Overheads of Thin Provisioning. In it, Hu makes a very interesting claim that wide striped LUNs have “greater protection from multiple disk failures”. On this point I have to disagree. Firstly, if a disk fails within a RAID group, then the impact on a LUN is only experienced if the subsequent failure is also in the same RAID group. This is a fact whether then LUN is wide striped or not. For wide striped LUNs which are spread across multiple RAID groups, there’s more chance of a failure because a double disk failure could occur within any of the RAID groups supporting the presentation of that LUN.
In addition, wide striping has more impact if a failure occurs. One benefit of having LUNs created from a single RAID group is that the impact of that RAID group failing is limited to only those LUNs. Imagine a 300GB 3+1 RAID group divided into 18x 50GB LUNs. Failure of that RAID group impacts only the 18 LUNs. So, wide stripe across 10 RAID groups – now the impact of any RAID group failure is 180 LUNs. Remember that’s any RAID group failure, which is much more likely as we have more RAID groups on which every LUN is dependent.
Finally there’s EMC and their free Virtual Provisioning – free that is on new purchases, not existing DMX-4 deployments. While laudible, this offering is less generous compared to HDS’ Switch It On promotion which offers free UVM, Dynamic Provisioning (first 10TB only) and Tiered Storage Manager on existing USP-V deployments.
Wide striping and thin provisioning are clearly becoming features where vendors are looking to differentiate their products. This must be vindication for the likes of 3Par who’ve had these features from day 1.
P.S. You can find two EMC blogger references to the free Virtual Provisioning here and here.
I think that's only true if your actual RAID groups are all the same size. Since double disk failures require both drives out at the same time if you've got less drives in the RAID set the lower the chance – much lower as you get below 6. I do agree with your post.
I think that's only true if your actual RAID groups are all the same size. Since double disk failures require both drives out at the same time if you've got less drives in the RAID set the lower the chance – much lower as you get below 6. I do agree with your post.