Claus Egge - Storage Consultant
There is no real dilemma between 2.5-inch hard disk drives (HDDs) and 3.5-inch HDDs, since a tiered and well-balanced storage pool will accommodate both, as well as solid state drives (SSD). A detailed analysis weighing up all factors of cost, performance, energy consumption, physical footprint and reliability would be the ideal way to choose a storage hierarchy for particular data storage requirement. But, in reality, customers’ choice of disk arrays owes a lot to historical decisions.
This can provide an inhibiting factor towards the acceptance of 2.5-inch HDD shelves and arrays. It will take time for customers affected by this to understand and accept the differentiated attributes of 2.5-inch drives.
Small is often good in disk drive engineering where vibration, power consumption, and overall I/O speeds all benefit from the use of 2.5-inch HDDs. Having more spindles in a shelf means more read/write heads can be simultaneously active. And while the bits per square inch number keeps on growing, the question then becomes what is the best-sized basket for my storage eggs.
Some people worry about their HDDs being too big while others dislike large fragmented clusters of smaller HDDs. The 2.5-inch form factor will grow in enterprise arrays exactly because its spin speeds and capacity levels offer greater choice at better price points. There is still a role for 3.5-inch drives, such as providing bulk capacity data storage, but expect this form factor to play a lesser role in high performance storage array applications.
Whichever format dominates in the future, the magnetic spinning disk still has a future ahead of it, although it is being complemented by faster (SSD) alternatives. However, the design of future disk arrays will be dictated by new error correction philosophies, because RAID runs out of steam as disk numbers and capacities increase. But that is a different story...
The consensus is that 2.5-inch drives will predominate in performance-focused storage array applications, with 3.5-inch ones being preferred for bulk capacity ones. There is a general agreement that RAID rebuild times on high capacity 3.5-inch drives is becoming untenable and that is a problem that needs to be sorted. This could mean that enterprise servers and storage arrays move to 2.5-inch SATA drives for bulk data storage while desktops stick with the 3.5-inch drives. Maybe they will go 2.5-inch too though, with bulk data stored at the other end of a LAN or WAN link. As ever, we will have to wait and see. ®
I've started recommending 2.5" drives for backup purposes (Time Machine, retrospect, etc) to friends. Often they have plenty of capacity for their needs and the benefit of a pocketable enclosure (tidier, might actually hide it away or rotate them) and no need for a power supply (tidier, cheaper to run) outweight the cost/capacity disadvantage.
MAIDs are for all intents and purposes standard RAIDs wherein the disks are "spun down" when not in use. That is 100% in the RAID card. The Intel RS2BL080 is my current favourite card. It uses an LSI 2108 chip. It seems to spin my disks down when idle just fine. (I believe you need MegaRAID 3.6.)
DELL PERC H700 and H800 card can also be configured for Spin Down.
Most vendors don't call it MAID. They just call it "spin down." Usually tout it as a power saving feature. I should point out that a single modern RAID card can be married to SAS expanders to provide truly a "Massive Array of Idle Disks." :)
Chris Evans' comment "data loss is only an issue if two drives fail in the same RAID group" is a bit worrying coming from a professional storage consultant. RAID 5, 10, fair enough, but RAID 6 and 60 will keep on rocking with two failures, or four in the case of 60
Another benefit of 2.5 HDD is they have much higher MTBF, about a third higher than 3.5 drives. Mind you, thats if you place any faith in them, I'm more of a subscriber to the "MTBF=Meaningless totally bullsh*t figure" theory, but to be fair the lesser head movement should definately translate to longer life