There I was at a VMWARE event happily minding my own business getting up to speed on the major IT trend that is virtulisation. This was not really Oracle related, and ASM was very, very far from my mind.
However it seems I’m never very far away from ASM, as the event was run in conjunction with Pillar Data Systems and inevitably the fact I’m DBA came up in conversation and it turns out Pillar have added a special feature to their Axiom range of arrays all to take advantage of the ASM 1MB stripe depth.
The Impact of Stripe Size
Lets first consider what a typical 128K stripe depth would be like when writing a 1MB ASM coarse stripe:
So it requires 8 128K writes to write a full ASM 1MB coarse stripe, and any read of a 1MB ASM extent will require the participation of all the drives within the stripe set.
It’s not too difficult to imagine this may not be the best bet for scaling the number of concurrent requests. Of course with the 1MB stripe depth we have the following:
The reason being is that it strikes the correct balance between ensuring disks are spending more of their time transfering data compared with the seek time, but at the same time allowing multiple drives to come into play.
Another interesting feature of Pillar Data systems is it’s ability to have multiple stripe depths within a RAID set. This enables you to store your redo, and control files with the fine grained ASM stirpe of 128K in a stripe depth of 128K.
XIV – ASM in hardware?
IBM’s XIV Storage came on the market last year and to me seems like it contains a lot of the features of ASM but done in hardware.
Each logical volume within a XIV array is divided into stripes of 1MB in size. XIV also uses a fairly familiar mirroring algorithim in that copies of the 1MB chunks of data are kept on 2 independent physical devices. Does this sound familiar to ASM users? It should.
These stripes are spread over all disks within the system
Like ASM it does not use traditional mirroring and there is no conept of devices being mirror pairs of each other.
Not that ASM needs much vinidication but they do say that imitation is the sincerest form of flattery, but clearly the ideas behind ASM have been found to be good ideas by other vendors as well.
Of course these stonking great arrays don’t come cheap and at least with ASM you are getting some of this redundancy for free with your Oracle database license. Though it would be nice if there was some snapshot or clone features included in some future ASM release.