SAN expansion - 45 10K drives or 30 15K drives

[jsnavely]

I know there's no clear answer... just thinking out loud.

My budget will allow me to purchase 45 10K drives in a 2G backplane OR 30 15K drives in a 4G backplane.

My ultimate goal is to support the largest number of guests (at acceptable performance). I would likely run the 10K drives as Raid10. I'm undecided if I went 15K. I guess I feel like 10K leaves me in a better position short term. Should I sacrifice a little now to begin a move towards 4G? This is a 4Gb/s fiber SAN.

For the record, the 30 15K drives are actually cheaper, but I will not be able to reallocate any money I don't spend.

[raadek]

Tricky question

As 15k vs. 10k delivers somewhere around 30%-40% more performance for random workloads, your comparison gives roughly the same result for both scenarios.

But there are some additional considerations around this, like usable capacity (are we talking about the same disk size in both cases?), array scalability (number of free disk slots left), etc.

Due to disk physics you MIGHT be actually better off with more 10k drives IF the 15k ones are of the same capacity. More drives will be less "filled in", so more data will reside in the outside tracks, which tend to be faster than those closer to the disk centre.

But your mileage may vary...

Rgds.

[Anders_Gregerse]

There is also the green factor (power, cooling, space). I would go for the 30 15K drives, they would allow be to add 15 more drives when the budget allows it.

[jsnavely]

- Disks are 146GB in both cases

- We are not in danger of hitting array limits so that is not really a concern

- While power/cooling is not a limitation, it would be nice to use a little less power.

[williambishop]

You have less space with the 15k's, but you have a 4g backplane, which is preferred if possible. Would I make a correct guess if I said IBM? I doubt you save much money, as the 15k's use more juice, but maybe a little is better than nothing. You won't get as many drives, but they will be higher performance, and in a faster array(you can bottleneck a 2g backplane without a lot of effort depending on your workload). My personal choice would be the 15k's.

[christianZ]

I don't know what storage are you using - but remember each storage have a max. optimum of disks - all above causes more overhead and maybe the 45 disks are above it - then the option with 15K disks should be better. In this case you get the 4G backplane too and would be ready for 4G disks (when not available for now)

[ngrundy]

This may or may not help you here.

We run, currently ~150 VM's on 14 300GB 10K FCAL disks in RAID10. Our SAN shows performance for those disks hovering around 30%-40% spindle utilisation per disk during peak workload.

The 150 VM's include all of the boot disks, around 120 of them are Win2k3 servers, the rest Linux with a couple of Netware systems.

We keep things like DBMS disks on a couple of small 6-8 disk 15K 146GB RAID10 raid groups, these RG's do less than 15% spindle utilisation. File servers are housed on large multi terabyte SATA LUN's.

No Idea how the backplane thing works in you're respect, our unit is twin controller, with 4x2Gbit FCAL ports, each tray contains 4 2Gbit loops, two per drive. disks are dual attached.