Not unless SSDs become faster than active memory...

Don't forget that you can always use a RAM disk if you really want to. You might consider that to be volatile, but there are various ways to deal with that. You can use the RAM for reads and have writes go to disk (high IOPS on reads, low IOPS on writes), or use asynchronous disk backing for the RAM disk (disk may lag behind RAM).

Interestingly, Anand is claiming that he expects Intel's next series of SSDs to include a 600GB option at a bit above $500, which is currently where the 160GB drive costs. If this happens (and I'm not so optimistic as Anand), the cost per gigabyte will have dropped by a huge amount.

How? Leaving it to the kernel by tuning dirty_writebacks and other related knobs? LVM/Raid with ramdisk in the matrix?

For the RAID-1 approach, all writes would be synchronously written to the disk. To get it working, you need to mark your "real" disk with --write-mostly, which tells the kernel that that disk is to be used for writing, and the other disk should be used for reading. See here: http://lkml.indiana.edu/hypermail/linux ... /1153.html

The advantages of a RAID-based approach is that the RAID rebuilding process handles the automatic population of the RAM disk; the ramdisk device is re-added to the array each boot (although I think you'd have to script that?), and the system rebuilds it and keeps it in synch. Does a lot of the work for you.

This might be a valid reason to bump up the RAM in your linode, if you intend to do it. But it's not cheap, the cost for storage is $56.75/GB. But considering that Intel enterprise SSD storage costs $12.90/GB at retail, you're looking at a an effective markup of only 4.4x.

Linode marks up traditional disk storage by something like 20x (10x if you consider RAID), so 4.4x is not bad.

Thanks.

But, I'm curious. Isn't the very concept of VM on Linux geared exactly toward that, with all available memory used automatically for fs page cache? In fact, it should be even better than a RAID setup, because the nature of it allows more data in total, than what is currently worked on, especially in high read-to-write ratio situations.

For example, say you have 10GB of total data on disc, but at any given moment only 1GB is actively worked with. That would mean 1GB of hot cache data, the older pages being replaced with newer on-demand. That would also mean you need at least 1GB of RAM available for the cache. True, if your reads are spread randomly across entire pool of 10GB, there is no advantage here, over the ramdisk.

But if you have enough RAM for VM cache for entire data, then I don't see how ramdisk would provide any advantage. In fact, relying on VM cache alone you have a more stable system which will simply drop old caches in case some other process starts requiring RAM.

Or am I missing something?

Ah, but you're also arguing against the utility of SSDs

The only real difference is whether things are cached before or after the first access. And you can entice the OS to cache files in advance by reading them.

Heh, no, I'm not arguing against SSDs, I'm just questioning the reasoning behind using ramdisk/HDD raid array over what the kernel already does, and does better - except, as you suggest, first access situation, against which the ramdisk/HDD array might have advantage, but such advantage is not always necessary, depending on the kind of load of course.

By the way, I haven't seen so far one very important bit of math, in the price per GB calculation. What about heat and electricity?

Observed in a period of, say, a year, there is certain cost inherent to discs:

Price per GB + kWh electricity + kWh in required cooling = total cost

I don't have numbers (google suggests no meaningful numbers on the subject), but it seems logical to me that SSDs should draw less power and produce less heat. The question is, is the difference significant enough, and when observed in a certain period of time, does that reduce the total cost for the discs?

It depends on what kinds of disks you use. Are you comparing equal performance, or equal power?

I'll perform a very lopsided comparison and compare an Intel SSD to a 15K RPM 2.5" drive, which would probably be the fastest magnetic disk:

Intel x25-m (160GB):
Read/write: 0.15W
Idle: 0.075W

Seagate Cheetah 15K.7 (300GB, calculated from amperage on +5V and +12V in 6gbit mode):
Idle: 11.62W

Clearly those 15K drives are power hogs, they use like 5x more than a 2TB WD GP drive.

Power difference: 11.545W saved
KWh per year saved: 101KWh
Cost per KWh in Montreal: $0.0545 (residential rate for first 30KWh per day, business rates are slightly higher)
Power savings per year: $5.50
Price difference between drives: ~$162

Number of years before SSD saves money on power: ~29

But if you want equivalent capacities, the SSD will take centuries to catch up.

This isn't really representative of a datacenter. Datacenters have to size their batteries, generators, and cooling for the power they supply. And electricity's cheaper in Quebec than it is in a lot of other places.

True, if you go by DC charges... they're only a few times higher. I'll use UberBandwidth. $17 per month per amp. That's $17 for 86.4 KWh, or $0.197 per KWh.

That's $19.90 per year saved, or 8 years just to break even. That's too long to amortize hardware. If you were trying to match capacities, I believe the price difference would have been $662, so it would then take 33 years to break even.

My point is, the power savings from an SSD aren't really a big concern unless you're comparing IOPS. At that point, it takes 18x15K drives to match the performance of an Intel x25-e, and the power savings become tangible. But if you're talking about matching the capacity, or matching the number of drives, there's no real cost advantage.