modular vs. cache-centric disk arrays

[Vladimir Vybiral]

Hello, just a short word of opinion:
Today, the cache-centric arrays offermore or less the same levels of features (remote mirroring, speed, availability, heterogeneity, you name what) as the modular arrays do.
Differences:
Cache-centric - Management and configuration is often tedious and involves sometimes vendor engineers to come to Your site
Modular - if we speak about modular example of the EVA5000, the management is EASY and done in minutes, plus the array intelligence does some things by itself (if You trust it - but I would) - like load levelling, hot sparing, cache allocations etc.

Cache-centric - The initial cost is HIGH, as You need to buy the engine, motor of the array, which is itself a box that occupies some space in Your serverroom. Need more space? Call engineers, let them add drives, init them, init containers, format, include, create disk groups, create mirrorsets and strip sets, create LUNs... You know this take time and a mistake means starting from scratch...
Modular - buy what You need today - a set of controllers and disk shelves, put them in a rack, hook and run. Need more space? Buy more disks, plug them in, and the space is automatically claimed, go to management, add the disks into a disk group of ANY size, data is automatically levelled (for more spindles = more speed), select a LUN a grow it by 100GB... (EVA example)
Need more speed? Either by just plugging disks You immediatelly increase the speed (as You have more spindles and data are striped automatically across everything) or buy another set of controllers, hook them to You rack, put some disks behind...

Cache-centric - Need mainframe connection? Go for a monolithic array. Mainframe computers are the only systems today that do not have a front-end cache so You need to have the xxxxx cache on the array. All other systems (win, UX, Linux, etc.) have their own cache so for a well tuned system You typically do not need those huge amounts of cache on the array.
Modular - typically 1-4GB of cache, which is NOT_MUCH certainly, but is okay for the delayed processing of stripe/parity write/rewrite. If You want more, buy another set of controllers :)

Life is soooo easy with the modular virtual arrays like VA or EVA. This is what I prefer unless there is a need for ESCON/FICON connection. The modulars do not support mainframe.

Oooops, wanted to write this short, but it turned out to be a bit wordy... :)

Vladimir

[Duncan Edmonstone]

Thanks all, I think this is pretty much confirming what I already suspected... I've tried to abstract this out into a couple of diagrams - I know this is generalising a lot, bu this seems to me to be pretty much what the differences are architecturally - see what you think, and put me right if I am miles off!

Cheers

Duncan

[Mike Naime]

For the Compaq modular stuff.

The (Modular) HSG80 and EVA can have shared (Mirrored) cache between the controllers. Your drawing shows it on each controller with no interconnect. I would place the cache in the middle between the two controllers.

The HSG80 has 6 SCSI channels/shelves with up to 14 drives per channel/shelf.

The EVA has up to 18 Fiber based SCSI channels (Shelves) of up to 14 drives per shelf/channel.

Originally they had fiber loops in teh design. Now they are using a special brocade fibre switch to interconnect all the shelves and controllers. A SAN within a SAN!

The EVA supports 240 drives, but it will hold 252 drives. I have not figured out yet which slots you are not supposed to populate! :-)

[Vladimir Vybiral]

A short answer on the last point:
You can populate up to 240 slots in any shelve. They are a way of "autoselect". The ones that You leave out depend on You and Your config.
The back-end on EVA is still a loop, though switched. (the devices there are four Vixel loopswitches, not Brocade). The compatibility does not suffer (as it is not related to the front end, of course), and it is cheaper than Brocade. There is one "dual loop" per shelve or enclosure, switched, virtually forming two loop addressing spaces. That is why You max out at 240 drives (one loop is 127 addresses - some reserved -2 for each ctrl = 120).

BR, Vladimir

[Duncan Edmonstone]

Guys, I think your missing the point here... I'm not trying to understand the architecture of any one disk array, but the general differences in architecture between different classes of disk array.

Thanks,

Duncan

[Vincent Fleming]

Duncan,

IMHO, Your diagrams are essentially correct. There are variations on the theme from different vendors, but you've displayed a good understanding of the basic technology with those diagrams.

Good luck,

-Vince