Saturday, 16 January 2010

The fallacy of cheap disk space

At work my concerns for application disk usage/waste are usually met with a scoff of "Why bother? Disk space is cheap!"

This is a myth I intend to destroy in this post. It is true that disk space gets cheaper all the time but it is not cheap.

I'm not even going to step into the NAS/SAN area - this myth can be debunked looking at local storage alone.

When people say disk space is cheap these days they are probably thinking of their videos, mp3 and pr0n collection at home; enterprise-grade disks are another story. I chose the cheapest drives I could find to illustrate my point:

Desktop: 1.5TB SATA 7.2krpm A$133 A$0.089/GB (WD15EADS)
Enterprise: 300GB SCSI 15krpm A$489 A$1.630/GB (SFU300G10K80P)

The comparison already looks grim for the myth: the cost of a data centre-worthy disk is more than 18 times that of a "disk space is cheap" drive.

In the real world redundancy is required so, for your typical RAID1/10 scenario, the cost is 36 times more or A$3.26/GB. You could get a better cost, A$2.45/GB, with a 3-disk RAID5 volume but the recovery time for today's disks makes the alternative risky.

Then you need to backup your data. Regardless of your backup frequency and retention period you will need to buy more tapes if you have more data.

Assuming that 1 LTO drive is able to backup all your data in the alloted time frame, a minimal retention period and 3x 200GB LTO-2 tape (in use, on-site, off-site), you are looking at an additional A$0.57 per GB backed-up and a total cost of A$3.83/GB - MORE THAN 40 TIMES the cost of cheap disk space.

One could argue that this level of performance AND redundancy is not always needed - and one would be right. For instance, if your application data is cacheable you can split the data in layers of increasing cost/performance with something like Blackblaze's solution at the back.

Once we're looking for cheaper storage the "disk space is cheap" advocates have already lost the argument. In case we're dealing with the stubborn kind let's move from the capital to the operational expense of storage.

The following statement is fairly obvious but probably needs to be reminded when storage cost is dismissed as irrelevant: disk space is an asset that depreciates at the purchase price, not market price. Once you've bought 1TB of storage for $A3.26 per GB you're stuck with the entry until it's out of the books, no matter how much cheaper disks get in the future.

Your backup opex also increases. You area going to rotate and store more tapes. You will need off-line storage and transportation for more tapes. You get the picture.

If your data is mirrored/replicated to other sites your network costs also increase.

More: because every procedure takes more time your labour costs also go up.

And more: enjoy while local storage meets your demand - the picture gets much uglier with external storage.