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On 3/8/07, Ron <rjpeace@xxxxxxxxxxxxx> wrote:
>Exactly, and WAL services other purposes than minimizing the penalty >from writing to high latency media. WAL underlies PITR for example. Never argued with any of this. >Near-zero latency media is coming, eventually...and I don't think the >issue is reliability (catastrophic failure is extremely unlikely) but >cost. I think the poor write performance is not an issue because you >can assemble drives in a giant raid 0 (or even 00 or 000) which will >blow away disk based raid 10 systems at virtually everything. Have you considered what the $cost$ of that much flash RAM would be? >Solid State Drives consume less power (a big deal in server farms) and >the storage density and life-span will continue to improve. I give it >five years (maybe less) before you start to see SSD penetration in a >big way. It will simply become cheaper to build a box with SSD than >without since you won't need to buy as much RAM, draws less power, and >is much more reliable. Don't bet on it. HDs 2x in density faster than RAM does, even flash RAM, and have a -much- lower cost per bit. ...and it's going to stay that way for the foreseeable future.
yes, but SSD drives do not have to overtake hard disks to be cost competitive. one reason for this is that a SSD based system does not need nearly as much ram as a hard drive based system due to 99% reduction of cache miss penalty. Also, flash drives are falling faster in price than hard drives. Unless there is some new breakthrough in hard drive technology the price differential is going to narrow a bit. Ultimately (and this may be quite some way down the road, but maybe not), SSD will be cheaper than spinning disk due to lower cost of materials.
ATM, I can buy a 500GB 7200 rpm SATA II HD w/ a 5 yr warranty for ~$170 US per HD. 1TB HDs of that class cost ~$350-$400 US per. (...and bear in mind the hybrid HDs coming out later this year that offer the best of both HD and flash technologies at very close to current HD costs per bit.)
it is not fair to compare cost of 10ms latency hard drive to 10us flash drive unless you add in the cost of ram to get the systems to performance parity.
The 128GB flash RAM SSDs coming out later this year are going to cost 4x - 10x those HD prices...
4+ decades of history shows that initial acquisition cost is =by far= the primary deciding factor in IT spending. QED: SSDs are going to remain a niche product unless or until Something Drastic (tm) happens to the economics and storage requirements of IT.
Historically what you are saying is true but something drastic is indeed happening. There is currently a power crisis in many colocation facilities in the u.s. For example, for at least 1/3 of the cages in qwest at sterling, va are sitting empty because there are not enough circuits to give out (qwest is hitting environmental regs so that adding any additional generator power they would have to re-classify as a power generating facility). This basically puts their customers in a bidding war for available circuits and is making power draw a very significant factor of cost of rack rental. It is not unreasonable to factor in, say, one year of savings of monthly power bill at a facility. some facilities are so strapped they will only give out dc power circuits...
>Disk drives will displace tape as low speed archival storage but will >probably live on in super high storage enterprise environments. Again, don't bet on it. Tape 2x in density even faster than HD does and has an even lower cost per bit.
this is a completely different argument but disk based backup systems are exploding as are the companies that design and implement them. tape has problems that go way beyond performance and price. whizzing robot arms that stick tapes in and out of things may have looked cool and futuristic in 1960 but its time to put this technology to bed for good. merlin
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