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I'm in the market for a RAID card and really like Intel hardware. I'm looking to build two separate arrays:

  • One RAID-5 array consisting of at least 3-4 2TB 7200RPM hard disks.
  • One RAID-0 array consisting of 4 Samsung SSDs.

I'm aware of some of the differences between cards such as cache size (pretty straightforward) and connector count (not so clear). Thus, I have a few questions on how best to choose a RAID card.

What does connection count really mean? I'll be running all SATA III drives and I'd like some headroom to expand on the RAID-5 array if I ever need more storage. I'm confused by the SAS to SATA conversion. Does having 4 internal SAS ports mean that I'll only need to use two of those ports, one for each array with a converter to split each SAS cable into a bunch of SATA cables? Is SAS capable of transmitting the high read/write speeds of my wicked RAID-0 SSD array?

It seems that there's a huge range of price in the Intel hardware RAID series, but I can't seem to figure out why.

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Not enough for a answer, so I leave as a comment. You want to "Split" as little as possible so devices are not competing for bandwidth over a single link. So 4 SAS connections each with a 2:1 SAS to SATA connector is preferable to 2 links each with a 4:1 SAS to SATA connector. –  Scott Chamberlain Dec 13 '12 at 20:47
    
What's the theoretical throughput of an SAS cable? How easy are they to saturate? –  Naftuli Tzvi Kay Dec 13 '12 at 22:04
    
The terminology can be a bit muddy, but a SFF-8087 connector is an x4 connector, meaning the single connector houses 4 physical ports. At that level, each port corresponds to one 6 Gbps link. If you connect 8 drives to the RS2BL080 that you're looking at, using the included cables and no additional port multipliers or expanders, none of the devices will be sharing a link. –  rob Dec 13 '12 at 23:29
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With four 2TB 7200RPM hard disks you might want to consider RAID 10 over RAID 6 (same resulting capacity, but better performance). Downsite is that expansion to five 2TB drives would not be possible. –  Hennes Dec 14 '12 at 1:29
    
Yeah, for this array I'm mainly considering redundancy and storage space first, so I'd stick with RAID-6 so I can expand quickly and easily. –  Naftuli Tzvi Kay Dec 14 '12 at 2:28

1 Answer 1

up vote 1 down vote accepted

It doesn't matter how you connect the drives physically; you'll configure which drives go on each array in the firmware.

You may want to consider RAID6, as the likelihood of catastrophic failure with RAID5 has increased as capacity (and, as a result, rebuild time) has increased. RAID6 adds a second parity disk, so if a second disk fails before your rebuild completes, the array can still maintain data integrity. But keep in mind that RAID is no substitute for frequent backups.

Which models, in particular, are you looking at? Some support 6 Gbps, while others only support 3 Gbps. There are also other differences that affect price, such as cache (sometimes including the ability to use one or more SSDs as an additional cache tier), number of ports, battery back-up (BBU), etc. Many cards won't let you enable the write cache if you don't have BBU, for the purpose of data integrity.

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I'm definitely considering RAID-6, but it's important to ensure that I have the right amount of internal connections to support a 6TB usable array :) I'm currently considering RS2BL080 or something hopefully cheaper which can give me similar results. I'm going to be getting the BBU as well, but first I'd like to find a nice but cheap card. –  Naftuli Tzvi Kay Dec 13 '12 at 20:54
    
You can use port multipliers or expanders to attach additional drives, but all the devices attached to a port multiplier will share the same link. This will be less of an issue for spinning disks, but--as Scott Chamberlain said--it's preferable to configure your storage system to minimize the number of of devices per link. That said, depending on your disk usage patterns, you could put 2 or maybe even 3 spinning disks on each link without noticing any performance degradation. –  rob Dec 13 '12 at 23:23
    
Yeah, that makes sense, I'll run one SAS to four SATA III ports to the SSDs and one SAS to the HDD array. If I need to add more HDDs, I'll divide the links and share them. –  Naftuli Tzvi Kay Dec 13 '12 at 23:36
    
So now, the main question is between the 256MB cache and the 512MB cache models... just how much caching will I need... –  Naftuli Tzvi Kay Dec 13 '12 at 23:43
    
Without benchmarks of your expected real-world scenarios, it's impossible to say. You could spend the extra $70 on more cache, on an additional SSD configured as a caching drive (assuming you're already planning to buy the hardware key to enable the "optional premium features"), or you could call it good enough and save the money for now. –  rob Dec 14 '12 at 5:47

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