Hard Drives

[LukeC]

Hello,

I'm speccing up a server to be a virtual host with local storage. Will I notice much difference between 15k SAS Drives and 10K SAS Hybrid drives? And pelase can someone also explain the difference.

[synaesthesia]

Difference of what, the spindle speed (15k/10k) or "Hybrid" ?

 

Hybrid drives mix and match SSD storage with spinners - the SSD part acts as a cache for the spinners - look up the Seagate Momentus XT drives available for consumer machines.

You probably won't notice a huge amount of difference in those two examples, hybrids will make up for a lot of speed - stuff that is cached will be quicker than *any* mechanical drive on the market, period. How well that works on a VM host though, I don't know. It depends what you're hosting on it really - if it's just file storage then you may find you're better off with standard 10k SAS drives or even 7200k SATA drives (aids price and capacity).

If it's a mixture, then maybe use a couple of different RAID arrays - full SSD for where you need very quick access constantly and "slow" SATA spinners for file storage.

[tmcd35]

How do Hybrids behave in a RAID environment? Are they like SSD's - require TRIM support on the controller and recommended to steer clear of RAID5 due high number of potential writes on the error code? Or are they more like standard hard drives, albiet with a larger cache?

[synaesthesia]

A good question, one I'd like to know the answer to as well. I'd edge towards staying clear personally however I've seen some raid controllers marketed with phrases including support of SSD cache.

[twin--turbo]

Unless the controller specificaly states proper support of SSD then avoid. And I seriously doubt a hybrid is a good idea on a server platform, they are great when they can load the common files used on a personal laptop but if there is a large amount of varied traffic then the cache portion wil be changing rapidly.

 

Rob

[tmcd35]

A good question, one I'd like to know the answer to as well. I'd edge towards staying clear personally however I've seen some raid controllers marketed with phrases including support of SSD cache.

 

Just done a quick google and that appears to refer to using one or two seperate SSD drives as a large capacity fast cache for the main array of standard hard drives, rather than supporting hybrid drives with their own bult in SSD caches.

 

Also, SSD raid controllers appear to be predominantly RAID1 or RAID10 from what I can see.

 

I'm sure we had this discussion before, I can't remember the outcome. What would be better 5x15k drives or 10x10k drives? Depending on use/RAID level intended, I'd be tempted to recommend to @LukeC more spindles at a lower spin rate? But I think the conscensus is use 15k drive instead of 10k hybrids.

[dhicks]

I'm speccing up a server to be a virtual host with local storage. Will I notice much difference between 15k SAS Drives and 10K SAS Hybrid drives?

 

Your hardware RAID controller, if you have one, will have a cache too, and I can't imagine having multiple layers of caching, both unaware of each other, is a good idea. A cache miss in the controller's cache will be another cache miss in the disk's cache, so you just wind up with the overhead of all disk requests going through two cache systems before getting to the actual disk. It's probably better to go for a larger cache (i.e. more RAM) on your RAID controller. We went for 15k SAS drives for our new VM local storage, plus a hardware RAID card, although we did then supplement that with another set of 4TB SATA drives for larger storage volumes - we can assign either, depending on the application.

[pcstru]

Unless the controller specificaly states proper support of SSD then avoid. And I seriously doubt a hybrid is a good idea on a server platform, they are great when they can load the common files used on a personal laptop but if there is a large amount of varied traffic then the cache portion wil be changing rapidly.

Most drives have solid state memory caching and have done for many years. SSD's just give drive manufacturers an option for much larger secondary cache. The benefit will depend on what you are doing and how good the caching algorithm is. I'd think it would be unusual not to see any benefit from a cache in a 'normal' server environment but there are certainly situations where that is not the case.

[tmcd35]

Most drives have solid state memory caching and have done for many years.

 

While I knew this, I thought there was a difference in the setup/makeup of the cache and the associated alogorithms.

[pcstru]

I'm sure we had this discussion before, I can't remember the outcome. What would be better 5x15k drives or 10x10k drives? Depending on use/RAID level intended, I'd be tempted to recommend to @LukeC more spindles at a lower spin rate? But I think the conscensus is use 15k drive instead of 10k hybrids.

 

I'd bet it depends :-)! In general, the movement of the head across the platters will be much the same in the 10K and 15K drives, only the movement of the platter under the head is faster in the 15K drive (generally a rotational latency of 2ms as opposed to 3ms), whereas both types will tend to have seek time in the 3-4ms (average). So in the worst case there is no difference between the devices because the limiting factor is the same - the heads lateral movement across the disc. In practice 10K vs 15K would be an average difference of around 30 io/ps (140 vs 170 (ish)). That suggests in normal use you would be better off with 10*10K, for around 920iops, vs 5*15K giving 650iops according to VMWare iops calculator (Raid 1&10, 50% read). Which is OK for 'normal' situations. At the extreme will be applications which require to be fed frequent and large bursts of data found contiguously on disc platters. Those might well benefit from the higher transfer rate of the 15K platter but in a RAID array, that might be blown apart by the way the algorithm spreads the data.

 

Personally I'd plump for the 10*10K platters.