I'm planning on buying four WD RE4 hard drives and an areca ARC-1212 RAID card for my new 2010 Mac Pro. I'm guessing I will need something like the Pro Cable in order to connect the drives to the RAID controller? Does anyone know if this is correct and whether I would need anything else?
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What cables needed for RAID in 2010 MPs?
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alphaod
macrumors Core
I use this one; works great. Be sure to get the backup battery for the ARC-1212 as it does not come with one.
Very few cards come with batteries, and in some cases, a battery is listed, but not actually available (ATTO for example).
Ideally, a battery + UPS system is needed for such cards, but in a pinch (budget limitations or unavailability usually the case), go for the UPS first as it gives a chance to complete a task already begun, or shut down properly if it's going to take more time than the battery in the UPS will be able to provide power.
This is also important for situations where the cache isn't large enough to contain all the data to be written (card battery just can't handle this situation at all, and work still may need to be re-performed if it doesn't resume automatically).
Any sensible RAID controller should stop acknowledging writes back to the host system once its write cache buffer is full. It is perfectly acceptable to fall back to the time it takes disks to write in that context. Doing otherwise is being reckless with your data. Adding a UPS unit doesn't change that; that implementor is still reckless. There are likely other aspects which equally as jacked up (e.g., they are probably ignoring disk cache buffer semantics also to juice out a few percentage points on some synthetic benchmarks) .
To me, it's more to do with the software. That is, it may not automatically resume a process (i.e. batch run) if a power outage occurs for data that didn't get written due to the power loss. In instances where the cache was full, and process stalls if written correctly, hopefully keeps track of what was/wasn't completed, and will resume once power is restored without user intervention. But there's no guarantee that this will actually happen, so the cache completing the writing of it's contents is essentially worthless in such a case, as the entire process is incomplete. The only way to be certain is with experimentation with the application/s used.
In instances where automatic resumption of the process/es does not occur, the UPS will offer some assistance to pause (save data at some point for manual intervention later) or complete the processes being run. If resumption is automatic, it can save time by allowing the user to keep working until the power is restored (condition that the utility company will have power restored to the location prior to the UPS battery depleting its charge). Otherwise, if the battery in the UPS is exhausted (an no generator system is in place), then the user will have to wait for power to be restored.
There's also the protection it can offer for AC Undervolt conditions (brown-out, not full black-out) vs. directly off of the wall socket. I've seen more brown-out damage than surge damage by a long shot.
Not the software but the overall system. However, that has very little to do with whether the write back cache is enabled or not on the RAID card.
The battery backup on the RAID card is there for two reasons.
1. With a write back cache the card can "lie" to the host about whether have written a data block to disk. Hard drives and their caches can have same issue. Most don't have a battery or capacitor to cover up that lie if power disappears suddenly so in a safe oriented configuration that is turned off.
A write back cache on a RAID card isn't going to do much if there are moderate frequency of large (i.e., write sequences at least as large as the cache) bundles of data. (e.g., writing 500MB files when the cache is 100MB. )
2. For RAID 5 and 6 there are multiple read/write/write operations to be done to disk that should be done atomic but may not be if power fails. That's not data from the host that is data the RAID card is composing for its "magic". That isn't a write cache.
The reason why need UPS in these two cases is because the battery only protects the memory. What needs to happen is more disks writes so that the card can get rid of this transient state. Some new RAID cards have Flash storage on board so they can write this temporarily to flash even if the disks aren't around. But has to get written somewhere.
There are other "half completed" stuff in the OS and Apps too. However, a "clean" shutdown would be good if there was no RAID card at all. If you just had regular disks on a simple internal SATA controller it would be less messy to just do a shutdown once power goes away. Nothing particularly unique to the RAID card's caches are present in that issue.
Brown outs ... that is just crappy power. The RAID card isn't alone is being better off if don't feed it crappy power. Hardly any electrical element in the whole system wants to eat crappy power.