Why use mechanical disks when flash memory offer much better performance?

I love mechanical disks for their huge capacities, yet for performance they are no longer competitive. Why has it taken enterprises so long to switch to fash based storage?

Mechanical disks have moving parts (Spinning rust!!):

On mechanical disk time to service one random IO = average latency (half a rotation of disk)
+ seek time 
+ transfer time. 
Hence IOPS = 1000ms / time to service one random IO.




Flash storage has no moving mechanical components, flash is resistant to shock. They are made up of NAND based flash memory. NAND flash can be SLC, MLC or eMLC and these solid state chips have different charactericits, most important endurance of Program/Erase (P/E) cycles which limit the lifetime of the SSDs:

Flash storage consists of NAND chips and controllers:


Characteristics of different flash NAND chips:





Flash storage can be:
a) SSD disk (in example below with SATA connector) - easy transition as same form factor 
    and same interface as traditional mechanical disk drives
           

                    

b) PCIe based flash (much higher bandwidth than SSDs)
                   

c) Flash arrays engineered from ground up (in example below Violin Memory)

           

   





Here are some typical figures 2.5" SFF disks:

Device	Type	Rule of thumb IOPS	Interface	Example cost	Cost/GB	Cost/IOPS	Endurance
7,200 rpm SATA drives	HDD	80	SATA 6 Gb/s	$249 for 1TB Seagate Constellation.2	$0.24	$3.11	5 years*
10,000 rpm SAS drives	HDD	140	SAS 6Gb/s	$409 for 600GB Seagate Savvio 10K.5	$0.68	$2.92	5 years*
15,000 rpm SAS drives	HDD	180	SAS 6Gb/s	$349 for 300GB Seagate Savvio 15K.3	$1.16	$1.94	5 years*
MLC SSD drives	SSD	8,000+	SAS/SATA 6 Gb/s	$589 for 512GB SAMSUNG 840 Pro	$1.15	$0.07	5,000 P/E cycles
eMLC SSD drives	SSD	8,000+	SAS/SATA 6 Gb/s	$1788 for 200GBHDS SSD400M	$8.94	$0.22	30,000 P/E cycles
eMLC SSD drives	SSD	8,000+	SAS/SATA 6 Gb/s	$1099 for 400GBIntel DC S3700	$2.75	$0.14	30,000 P/E cycles
SLC SSD drives	SSD	8,000+	SAS/SATA 6 Gb/s	$2399 for 480GB OCZ Deneva 2	$12	$0.3	100,000 P/E cycles

Just for interest here are Dell's storage options:

All vendors now have competitive SSD storage options.

Flash storage will soon replace mechanical disks for most applications that require high performance as prices are dropping fast. See example below of 512GB SSD compared to mechanical disk:
 http://www.storagenewsletter.com/news/marketreport/when-will-ssd-have-same-price-as-hdd-priceg2
             

RAID Calculator:
www.vclouds.nl/wp-content/uploads/2012/04/RAID_spindle_calculator.xls