ShadowImage and the 21st Century
Last week I was talking to some guys who are responsible for a pretty large HDS SAN, when we got on to the subject of ShadowImage. More specifically, the use of ShadowImage in situations where you need more than three copies of any LUN <cringe>.
There was nothing out of the ordinary in the conversation - just the usual moaning over the complexity of maintaining more than three copies of any particular LUN severe limitation of only being able to keep three copies of a LUN. NOTE: I know that you can technically keep more copies (up to 9) by having level 2 copies hang off level 1 copies. But it is horrendously complicated, and restricted to the point that it is rarely attempted. And many who attempt it are scarred for life by the experience.
Anyway, Im getting side tracked......... So we were having this gripe about ShadowImage when one of the guys dropped in the comment that very recently they came within a whisker of making the switch to EMC. Fair enough, happens all the time........ However, interestingly, one of the things they listed in favour the DMX was the ability to maintain more than 3 copies of a LUN! NOTE: Im assuming what they told me about EMC is correct. After all, I know very little about EMC and the DMX other than it appears to be prone to double disk failures and data loss - sorry couldn't help myself on that one ;-)
Although Im sure the eventual decision, to stick with HDS, was based on either cost or existing investment or something like that, I do find it interesting that a large customer cited this restriction in ShadowImage as a possible reason to switch to EMC.
Ive talked to lots of people about this restriction in the past, and in fact I half hoped that either the USP or the USPV would come with a feature upgrade that would allow us to maintain substantially more (level 1) copies of a LUN. Obviously in relation to this, both the USP and USPV disappointed me.
Of course I know that my opinion counts for very little, if anything - heck I only work with the technology most days of the week so what do I know. But surely its another matter altogether when one of your big customers lists this as one of the reasons they recently considered dropping your flagship product in favour of your mortal enemy's!?
So now that we've established that an upgrade to ShadowImage is required in order to bring it in to the 21st Century, how hard would it be to implement?
I asked this a to an instructor around the time the USPV was released, only to have him squint disdainfully at me down the barrel of his nose as he attempted to belittle me in front of the rest of the class by categorically proclaiming this will never happen "because its so deep in the code" - like he'd ever seen the code, I think not!
May be its just me and my lack of programming experience, but surely it cant be that hard to change the code. After all, HDS already allow for up to 64 level 1 copies if you use COW. Oh and apparently EMC already do it fairly well with their product. Evidence again that it cant be that hard.
At the end of the day despite the good performance and reliability of ShadowImage, it is hardly enterprise when it only allows for three copies of any particular LUN. So close, yet so far!
May be the next release, the Hitachi RAID 700 I presume, will come bearing glad tidings? It would be a big improvement for a lot of customers!
Nigel (HORCM death detected)