Enterprise Computing: The Wide Striping Debate
Enterprise Computing, GestaltIT, Virtualisation — By Chris Evans on July 12, 2009 at 11:17 AMI’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.
Tags: EMC, HDS, Hu Yoshida, Martin Glasborow, storagebod, Switch It On, thin provisioning, Wide Striping
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10 Comments
Chris,
I think its worth pointing out two things -
1. The UVM is for 3rd party storage only. No free UVM if you want to virtualise HDS arrays.
2. The 10TB HDP amounts to very little in large enterprise accounts. 10TB is neither here nor there.
The risk you state very much depends on how the wide-striping is implementing and how the RAID is implemented. I’m sure Marc will be along to correct me if I’m wrong but in 3PAR’s implementation, the RAID is at the chunklet level and not at the disk level. This has positive advantages in the rebuild times but also a double disk failure is unlikely to the impact you describe. At least, that was my understanding of 3Par’s implementation.
It is one of the advantages of starting without a legacy architecture…
You beat me to it Martin. My understanding of 3PAR RAID as well is that it is at the chunklet level and not the physical disk level.
Clarification came out today that “Free VP” is available to all VP-supporing Symmetrix platforms, new or already installed. DMX3 & DMX4 must be running Enginuity 5773; all V-Max arrays support VP with Enginuity 5874.
Barry
Thanks for that. Are you able to say whether/when EMC will have something like Zero Page Reclaim?
Chris
Martin/Jason
Thanks. As you say, “legacy” architecture. Unfortunately sometimes you can’t fit new technology into old hardware…
Chris
Right now I can only say that reclamation of unused/all-zero space is just another common feature of thin provisioning. Likewise the ability to shrink a storage pool to remove/repurpose unused capacity (Hitachi DP doesn’t support this feature, V-Max VP does). Feature races are usually temporal – any feature advantage is generally short-lived.
And indeed, if/when V-Max VP supports zero space reclaim, it will inherently be more efficient than Hitachi DP thanks to the smaller chunk/page/extent size of VP (768KB vs. 42MB).
Can any of the above who bring up 3PAR RAID at the 256MB chunklet level offer any more information on how this is better than traditional RAID Groups for lowering the risk of double disk failures?
My understanding, albeit limited, is that it lowers the “possibility”, but that the possibility exists….
Also the 3PARs dont currently offer the ability to create storage pools. Its all your eggs in one basket scenario with an albeit lower risk of double disk failure. However, other technologies offer the ability to create multiple pools and spread your eggs among multiple baskets.
Im well aware of the limitations in the HDS implementation but am not so sure that its all that worse than the 3PAR implementation.
Thoughts??
So Barry,
In saying “..And indeed, if/when V-Max VP supports zero space reclaim, it will inherently be more efficient than Hitachi DP thanks to the smaller chunk/page/extent size of VP (768KB vs. 42MB)…” Are you suggesting that VP ZSR will be less efficient on V-Max than on 3PAR as 3PAR has an even smaller chunk/page/extent? Not as simple as that otherwise we’d probably all have 1K extents (OK obviously not but its not all that simple is it).
And interesting that none of those extolling the virtues of 3PAR above have piped up with explanations of why what 3PAR does is better than what the likes of HDS and EMC do. Hmmmmmmmm! (or may be they dont follow the comments feed on here) No probs as Im pretty sure I already know anyway…….