Barry’s question

7 01 2008

Via email:

“When you are thinking about Disaster Recovery, CDP, do you assume that Tier3 is adequate, mainly because this is backup only, or maybe DR so hopefully not needed? How does your thinking proceed? Do think about your primary data at the same time?

I ask this as a loaded question, knowing that anything that has to copy to, snap to, or mirror with, secondary, backup or DR or CDP storage now has a definite tie with the primary.

Barry Whyte

SVC Performance Architect
IBM Systems & Technology Group”

This is a loaded question! To start with, I’ll note some assumptions and concept clarifications to ensure we’re talking about the same thing- if I’m off on anything, let me know ;)

  • CDP: continuous data protection, an IBM backup software algorithm- small changes sent to a central server continuously
  • Tier 3: low price random access storage media- not tape, usually cheap SATA drives
    • Note: there’s been discussion about these tier definitions before, and I hold that tier 3 means different things to different companies.

To your question- I would have to decide based on the company’s current architecture. If they have a storage solution that has synchronous mirroring between two sites, then using low performance drives on either side will slow production. If they’re doing asynchronous replication (or a server instead of storage based DR solution), I would probably be fine with SATA/tier3.

To explain my reasoning, I must first say that I can not decide without having a specific case and a IT person to question. My advice would be based on risk tolerance versus capital expenditure tolerance. Secondly, SATA has a undeserved bad rap- the drives are about as reliable as other enterprise ones (according to Google). SATA drives are certainly not fast for random access loads, but for sequential and low urgency loads like backups, they will do the job.

Low performance media will always be part of a healthy storage balance- the most bang for most companies’ bucks will be in prioritizing their applications (or even their data), and using the media that makes the most sense. Need an Oracle server to stop freezing up your warehouse management app? Put that baby on 15,000 RPM FC hard drives- lots of them. Need to keep a backup copy of a file server on site in case of a server outage? SATA will do the job. Need to keep nightly point in time backups of your entire storage infrastructure for years? You probably can’t afford to put that on drives at all- use tape.

That said, most companies that haven’t reached a boiling point in their storage gear yearly expenditures won’t bother to do much of this stuff. Face it, tiering your applications for storage takes operator time, and gear just seems to feel cheaper to management than IT man hours. That and the explosive growth of media density in the last 5 years have kept tiered storage plan adoption either to the ridiculously large data producers who have no other choice (like large banks) or to more forward thinking smaller shops.