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Is it sane to build a machine with RAID 5 expecting to hold around 5 Tb (and able to grow) with 1.5 or 2 Tb SATA II disks?

It will be used as audio and documents storage. I already have chosen a chassis with 16 3.5" hard drive bays. Now I'm thinking about what to put on those bays.

Pros I see:

  • Cheaper per Tb
  • Greater extensibility per chassis (3 times max capacity using 1.5 vs. 0.5)

It's the cons I'm wondering about. I expect using big disks to be slower, but is this really the case? I don't know about failure rates between small and big drives, are big ones expected to fail more often?

So, two questions:

  1. What are the cons of building a 5Tb RAID5 with big disks?

  2. Any particular drive model you'd recommend for either big (2Tb) or small (0.5Tb)?

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Bigger drives != slower performance. The drive platters aren't physically bigger, they have more platters. Each surface of each platter normally has it's own read\write head (if each surface of the platter is used for data storage) so access times should be roughly equivalent to smaller drives. –  joeqwerty Oct 20 '10 at 15:21
On the contrary, I would expect bigger drives to be faster than smaller drives. All else being equal (rotational rate, number of heads, physical size), a larger drive will have a greater bit density than a smaller drive, which implies that its read/write rate will be higher. –  Steven Monday Oct 20 '10 at 15:47
It depends what you mean by 'big' and 'faster' - at any point in history the largest available capacity drives have always been pretty slow compared to the drives built for high load or outright performance (see today's 2TB 7.2krpm disk vs. 600GB 15krpm disks for example). Certainly bit density can help but so does rotational velocity, cache, interface speed, request latency, queuing capacity and a number of other factors. Either way rebuilding R5 arrays with consumer 1/2TB disks takes AGES and slaughters the drives during that time. –  Chopper3 Oct 20 '10 at 16:41
Seeks are often slower on the big commercial drives because it takes longer to find the right data track. –  Zan Lynx Oct 20 '10 at 17:15
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3 Answers

up vote 8 down vote accepted

You had me a R5 - don't.

The reason is that in the event of a disk failure you have zero protection until you've replaced the disk and the array has rebuilt.

For large cheapo SATA disks this rebuilt process can take DAYS - meanwhile you are at the mercy of a second disk failing - at which point it's game over.

Also this type of disk is rarely happy to work solidly 24 hours a day and I've seen rebuilds kill disks - again making the whole thing rather dubious.

If you can use RAID 10 over 5 or 6, if you insist on 5/6 then use 'enterprise' disks capable of a 24/365 duty cycle.

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Are the disks that advertise "works 24/7" really trustworthy? For example seagate.com/www/en-us/products/consumer_electronics/sv35_series –  Vinko Vrsalovic Oct 20 '10 at 15:05
@Vinko, No. the million plus MTBF is nonsense. (and IMO seagate is the worst.) That said, you "cannot not" buy them. How do you explain when a drive fails that you bought the desktop grade. So you are stuck buying paying extra for enterprise drives. –  Leo Oct 20 '10 at 15:11
REALLY, REALLY NOT - those drives are for PVRs like Tivos etc. (in a bizarre coincidence I happen to buy literally hundreds of thousands of those disks, but I digress) - they'll SPIN fine for 24 hours a day, try thrashing it all day long and see it's MTBF go through the floor - I'm thinking more like these; seagate.com/www/en-us/products/enterprise-hard-drives/… –  Chopper3 Oct 20 '10 at 15:12
Is a 500GB enterprise class HD safe enough for RAID5? (that's the other alternative for me) –  Vinko Vrsalovic Oct 21 '10 at 16:54
@Vinko, it's as good as you can get but most, if not all, sysadmins on this site generally hate R5 –  Chopper3 Oct 22 '10 at 10:46
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Using RAID5 with this configuration, you introduce additional risk to your RAID. With the storage density introduced with modern disks, the likelihood of encountering a bad block on multiple disks is higher.

If using 1TB+ disks, it's recommended to use a RAID6 as opposed to RAID5, as it has an additional parity disk.

If you want greater speed as well as better guaranteed availability, you might consider RAID 1+0 as well.

As far as speed, compare the seek time and various other specifications. Speed does not vary much with commodity disks that are higher density. When I bought disks a few months ago, the largest tier1 SAS disk I could buy was 450GB.

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+1 - The bit error rate of disks makes errors a virtual certainty with large disks, as opposed to a theoretical risk. Very large RAID-5 sets carry with them the risk that you'll have a bit error during reconstruction. RAID 1+0 is the way to go for performance and availability. –  Evan Anderson Oct 20 '10 at 14:49
@Evan: That's all well and cool, but the cost is too high, especially when (as is my case) performance is non critical. So I guess RAID 6 it should be –  Vinko Vrsalovic Oct 20 '10 at 14:54
what's the cost of losing the lot? –  Chopper3 Oct 20 '10 at 15:02
@Chopper3 That's a fine argument. Let's see if I can make it fly. –  Vinko Vrsalovic Oct 20 '10 at 15:06
RAID != backup. You will have backups, right? –  Steven Monday Oct 20 '10 at 15:55
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Short of replacing the whole array, you can't grow/extend your array with raid5.

I'm using mirrored pairs, you don't get a boost in write performance, but you can easily extend your volume by adding pairs. It's not a performance solution, but it provides protection from disk failure.

Oh and for a budget array, there's an obeservation I heard years ago... what happens when you buy a stack of identical disks, at the same time, transport them to the same computer and build an array where they all run for the same duration, under the same load, under the same enviromental stress? What's the likelihood that given one has failed, that another isn't just about to fail?

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I've successfully grown RAID5 using Linux software RAID. –  Warner Oct 20 '10 at 17:15
You can expand a RAID-5 with many controllers/software solutions. –  phoebus Oct 20 '10 at 19:39
I stand corrected, short of rebuilding the whole array, you can't grow/extend your array with raid5. mdadm has a trick to rebuild the array on the fly, so the point is moot. It looks like the commercial solutions do it too. In the past, to expand a raid-5 array, I had to replace all the disks in the array with larger ones. –  mgjk Oct 21 '10 at 18:30
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