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38049
August 16th, 2011 05:00
Is RAID 10 really that great on an EqualLogic?
I have a brand-new, not-yet-live PS6000XV (16 x 300GB 15K) which I'm just starting to get to grips with. It will run a SQL Server DB (ever-expanding) which currently runs at around 600GB in total - this includes all the varieties of backups, logs etc. The usable capacity of our SAN is 1.8TB roughly, in a RAID 10 setup. We have plans that will expand the dataset quite significantly - we will easily be at 1TB by Christmas so we'll be looking to get another member early next year.
So, on to my question...as space is already a concern (and those with the funds want to hold off on laying out on another SAN for as long as they can), I'm looking at RAID 50, to give us around 3.5TB instead. Now, I understand the academic differences and the advantages and disadvantages, but I'm struggling to see how this applies in the EqualLogic world. For instance:
- with write-caching, are the differences in performance between RAID 10 and RAID 50 that pronounced?
- I've been advised that it's possible to migrate from RAID 10 to RAID 50 on-the-fly, what's the performance impact of this, and how long would it be expected to take?
- calculations I've done suggest that write performance may actually be better under RAID 50 - again, is this a major factor given write caching?
So, to summarise, the conventional wisdom is that RAID 10 is the gold standard in terms of performance and rebuild time, but I just want to question this. Are the relative disadvantages of RAID 50 really that bad as to outweigh a space increase of around 70%?
I'm not looking to start an argument here, given enough space the easy choice would be R10...as we're a bit limited though I really want to explore the possibilities of R50. Hope someone can help.
So, on to my question...as space is already a concern (and those with the funds want to hold off on laying out on another SAN for as long as they can), I'm looking at RAID 50, to give us around 3.5TB instead. Now, I understand the academic differences and the advantages and disadvantages, but I'm struggling to see how this applies in the EqualLogic world. For instance:
- with write-caching, are the differences in performance between RAID 10 and RAID 50 that pronounced?
- I've been advised that it's possible to migrate from RAID 10 to RAID 50 on-the-fly, what's the performance impact of this, and how long would it be expected to take?
- calculations I've done suggest that write performance may actually be better under RAID 50 - again, is this a major factor given write caching?
So, to summarise, the conventional wisdom is that RAID 10 is the gold standard in terms of performance and rebuild time, but I just want to question this. Are the relative disadvantages of RAID 50 really that bad as to outweigh a space increase of around 70%?
I'm not looking to start an argument here, given enough space the easy choice would be R10...as we're a bit limited though I really want to explore the possibilities of R50. Hope someone can help.
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JOHNADCO
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August 16th, 2011 08:00
Donald_Williams
72 Posts
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August 16th, 2011 13:00
The answer to such questions, is "it depends". Depending on the load you plan on expressing, R10 could be the most appropriate RAID level. Your best friend here is SANHQ. Download that from the website and that will assist you greatly in determining the current load and whether R50 would provide enough IOs. In fact a future version of SANHQ (in beta now) will actually attempt to show you what the impact of changing the RAID level will be.
Expansion time greatly depends on the load while it's expanding. You won't have access to the new space until the process is complete. Typically allow a day for the process to complete. Since it runs at a lower priority compared to IO requests from servers. Again the impact depends on the current load.
Re: Cache. Yes, even with cache, the RAID level is very important. When doing small random writes R10 is so much more efficient than R50/5. However, more large sequential reads really benefit RAID50. Because eventually, regardless of the cache size, data has to be written to the disks.
Regards,
Don
TimtheDBA
4 Posts
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August 17th, 2011 08:00
Currently the db runs off a 4-disk R10 setup; pretty much any arrangement involving 3.5x the number of disks is going to be a massive improvement. So everything's relative. I'm sure R10 will offer a performance improvement over R50 (still not sure how much) but I know a 7x2 R50 setup will perform massively better than a 4-disk R10. It seems adding spindles trumps any RAID config argument. So I'm going to go with R50 - I can't ignore the opportunity to get 3.5GB rather then 1.8GB.
Doubtless we will need to get another member at some point in the future, but the space gain now means we can put the expenditure off probably for a year rather than 3 months. And as long as the system performs adequately, my boss will like that!
Donald_Williams
72 Posts
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August 17th, 2011 10:00
While not a satisfactory answer it is nonetheless true.
If you haven't done so already, get SANHQ installed. Let that run for a few days and you'll get a good idea how hard you are currently hitting it. That will at least point you in the approximate area where you'll be with R50. I.e. if you are currentely showing a "low" load per SANHQ, you're likely to be safer going to R50.
You can also sign up for the SANHQ beta and check you're IO history and see more definiitely how it will be with RAID50. It's on the EQL support website. There's a link that you can e-mail and ask to be included in the beta programs.
Regards,
-don