Data Center Server computers and their disk drives

512 vs 520. 

Learned this one the hard way, didn't understand some details before I spent some money. Fortunately only a little bit of money. One of the sellers tried to warn me, but I didn't know what he was saying, and I didn't understand the situation yet.

There are several features/details about disk drives you want to understand before you buy any and end up with units you cannot use.

I am only addressing the machines I am using, so others may be different. Those machines are:

HP DL360 Gen 6/7/8/9

HP DL380 Gen 7/8/9

All of these machines have internal RAID hardware. Sometimes that hardware is a removable daughter board, sometimes its directly on the mobo; I have both. I have 410, 420, 440 model/version numbers. 

Sometimes the RAID software interface is an old-school text interface, specifically Gen 6 and 7, with a mousable GUI for Gen 8/9 (and presumably 10/11).

Your first criteria of concern is the physical size, either 2.5 inch or 3.5 inch, and the quantity that the machine can mount.

For the machines that will take 3.5 inch: comes in both 1U and 2U. You will need matching caddy trays that are 3.5 inch and match the machine. Again, Gen 6/7 are different from Gen 8/9+. Finding the right screws is important too, and the screws for a 3.5 inch drive are different from a 2.5 inch. Quantities that can be installed are 4 in a 1U server, and 12 or 15 in a 2U; imagine having 15 28-TB drives in your machine, that's 400 terabytes. (I have seen pictures of another brand 2U server taking 24 drives!)

For the machines that will take 2.5 inch platter or SSD: also comes in both 1U and 2U, quantities vary, from 5 to 8 to 10 to 16 to 24. Platter drives in 2.5 inch are three rotation speeds: 7200, 10K, and 15K. The most common is the 10K. Capacity goes from 36GB to 72GB to 146 to 300, 450, 500, 600, 900, 1.2 TB, and I think there's a 1.8TB unit. I have a few of nearly all those sizes. For a data center you really want the units from 500 to 1.2T. Most all those are 10K, which is good.

The faster a platter spins the faster you can read data from it. So 10K is faster than 7200, and 15K is faster still. Speed correlates against capacity: there are no 10K or 15K 20TB drives.

SSDs are 5-10X faster right out of the box. You really want to use them as much as you can, esp given that in the same physical size you can get up to 8TB per unit. They are a good bit more expensive. EBay will sell you a 500GB 10K 2.5 inch platter drive for $20, a 900GB for $25. These are pretty good deals until you need a lot of space.

All these details you can read on the device label.

The thing you CAN'T read on the label is what the bytes-per-block/sector number is. There are several possibilities here, 512, 520, and I have one drive that is 4096. (Later: I discovered that I have some that are 4160, whatever THAT is)

My machines, the HP DL360/380s, will only take 512s. The 520 is an attempt to squeeze a teeny bit more data onto the disk, the 4096 is going to be a bit wasteful but that drive is 10TB so you probably don't ever notice that. These get used in other places like a SAN, which is all about capacity, or some form of JBOD.

512 vs 520 

Before you click Buy It Now make sure you know the answer to this, else you just bought a brick.

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That said, it is my casual understanding from reading online that it is possible to convert one to the other, but the process sounds a little complicated, and it cannot be done on my machines.

What I have read of that conversion: first off, standard RAID on my machines cannot do it, and will barely even tell me about it. Install one, go to the Gen 8/9 gun-based Smart Storage Administrator, look at details on one of the physical drives, the drawer window on the right will say something like "can't be used for RAID" and will maybe also say "520". So you are going to have to take this drive to another machine that has a daughter card (or PCI) that is itself configured for HBA mode, not RAID, so that software will have more direct access to the drive, then some other software tools (ShredOS apparently) will allow you to turn a 520 into a 512. I have never even gotten close to trying this. One of these days, maybe...

Later: I have received a PCI card for a standard PC that is an HBA adapter that suppposedly will let me plug in a drive, talk to it directly, tell it to reformat to 512. Apparently that will take some hours or overnight for bigger drives. Also part of the picture: you boot something called ShredOS from a thumb drive, it uses the HBA card to allow talking to the disks. Story is you can do several at the same time.