On Thu, Apr 05, 2012 at 11:57:05PM +0000, James Harper wrote:
According to the HP Configureaider:
300GB 2.5" 10KRPM SAS - $422 300GB 2.5" 15KRPM SAS - $772 200GB 2.5" MLC SSD SAS - $4696
that SSD price alone is *precisely* why i'm dubious about the claims of enterprise vendors. there is just no valid or justifiable reason why an SAS MLC SSD should cost anywhere near that much. I can partially buy the argument that higher quality magnetic disks have substantially increased manufacturing costs, but not to the point of believing that they're worth it. SSDs have no such manufacturing-cost excuse (higher quality parts may justify a few percent extra, perhaps as much as 5% but the manufacturing processes and tolerances would be the same). i think it's also subverting the point of raid (to have an array of cheap disks so that one or more disk failures won't lose your data) for nothing but commercial gain for the vendor. more to the point, i think it's shameless profiteering from peoples' fairly natural CYA motivation, which leads to decisions to buy the 2 or 5 or 10 or more times as expensive "enterprise"-labelled product even though it's only a few percent better or a few percent less likely to fail. this is compounded when consultants get involved - their percentage markup on overpriced goods is far fatter than the same markup on reasonably priced goods. thus they are motivated to keep the gravy-train rolling. (OK, this is only an issue for me personally at work where i'm sometimes only allowed to use such absurdly overpriced gear. it's not my money, so i suppose i shouldn't care....but it offends me to see so much money wasted on so little, when the money could be better used for other projects or other things. or, worse, to see a project completely derailed because someone higher up wants to enterprisify the project and it dies or gets lost in a budget approval committee - same thing - because it has changed from costing a few hundred or a few thousand dollars to tens of thousands). IMO there are two key words required for successful raid - 1. "cheap", 2. "lots". "expensive" contradicts both of them. sure, if the price difference was reasonable (say 10, 20, or even 30% more), i'd choose the "enterprise" drives over the cheaper ones. but the difference is not reasonable. not even close. consider, for example, the 300GB drive for $422 above. to get a raid6 array of 3TB you'd need of at least 12 of them. $5064. To get a raid array of 3TB in consumer SATA disks, you could have 2 x 3TB @ $200 in RAID-1 ($400), or 5 x 1TB @ $95 in RAID6 ($475). Both options cost about the same as a *single* 300GB 10K SAS drive. you can also add more identical drives in RAID-1 for more speed and redundancy (e.g. 12x$200 = $2400, half the price of the 12-disk array of 300GB SAS disks), or add them for more speed, capacity, and redundancy in RAID6. in the 12x3TB raid-1 example, it doesn't matter if 11 of those cheaper drives die. you've still got a complete copy of all your data on the remaining drive. you also get the speed of all 12 disks for writes (vs the speed of only 10 for raid6, 2 are lost to parity) for raid6, well, you can only afford to lose 2 drives so buy some extra hot and cold spares. so, comparing 12x3TB SATA raid1 vs 12x300GB raid6 - is it really worth paying more than twice as much for 2/11th of the redundancy and 5/6ths of the read performance (and significantly impaired write performance)? or if you were using 12 x 240GB mid-range consumer-grade SSDs, you'd get 80% of the capacity and *thousands* of times the performance for half the price of the 10k 300GB SAS drives. and that's comparing against the price of only 10K rpm drives. The figures are much worse for 15K rpm drives.
I'd be careful about sticking your $860 SSD into a server if you require any sort of write performance or durability, you might find you get just what you paid for.
yep, pretty much identical performance and reliability without the "rip me off, i'm an idiot" price tag. also note comments about raid and redundancy above. i can afford to buy a lot more redunancy if the unit prices are cheaper. btw, resyncing a raid array of SSDs is far less likely to overstress and kill the remaining drives than resyncing an array of magnetic drives - no moving parts, random access, not as heat sensitive, much smaller time window until the array is fully synced, etc.
(*) a large part of the point of RAID is that it is a Redundant Array of *Inexpensive* Disks. enterprise drives fail on that particular point. the disks are meant to be cheap and replacable commodity parts.
... or Redundant Array of *Independent* Disks, as if the name tells you what sort of disks you should be using anyway.
Russell's already addressed that. It IS supposed to be Inexpensive, but enterprise vendors have distorted that for their own benefit...apparently with some success.
At 15KRPM you can read a single track in half the time, and therefore twice the speed.
with an SSD you don't need to wait for the head to get around to the right part of the disk to start reading again. read as many blocks as you want from wherever you want without rotational or head-movement delays. and without the heat caused by the platters spinning at such high speeds, or the extra power consumption of same.
You would also get additional gyroscopic stability although I don't know if that makes a difference in reality... someone (on LUV I think) mentioned that consumer grade disks performance suffered much more when placed in an environment with vibration (eg adjacent to other seeking disks in a server).
*ALL* mechanical disks can and will suffer from vibration problems if the environment they're in or their mounting is sufficiently bad. SSDs won't, of course. craig -- craig sanders <cas@taz.net.au> BOFH excuse #106: The electrician didn't know what the yellow cable was so he yanked the ethernet out.