Security
by miketalonnyc
Agreed that RAID technology is nothing new to those who have lived in the IT world for some time. However the CFO who's never had to worry about anything outside his laptop can really use this information to justify the budget for next year's tech development =)
Mike Talon
by mmelb
by sussex pete
by barb
This was hardly worth an article!
Instead, try answering a tough one, like how to duplicate the Merrill-Lynch trick (back on line four minutes after the WTC went down) without building, equipping, and staffing a duplicate data center! With hundreds of millions of dollars at stake, their budget must have been practically infinite; mine is not!
by miketalonnyc
Often in my dealings with clients I am asked to provide the impossible. "Give me absolute fault tolerance and high availability of all my systems with no budget."
I'll be the first to tell you it cannot be done.
I can give you Disaster Recovery - meaning that Recovery Time is rated in days, not minutes.
I can give you Data Protection, and depending on how much you have to spend that can be hours or mere seconds out of sync with the live data. It can be synchronous too, but we're talking small budget here.
I can even give you high-availability for your data-systems, but with severe limits imposed by bugetary concerns.
So you can have a DR plan, you can even have HA, but you can only have what you are able to budget for, just like anywhere else in the business world. If you want Enterprise Class DR and HA, you have to have an Enterprise Class infrastructure and budget to support it.
ML has the buget and infrastructure to support the level of High Availability they required to recovery from 9-11 in the time they did. Not every company can afford those systems, and my goal is to offer solutions that fit mulitple budgets, and multiple levels of DR needs.
Sorry for the marathon post, perhaps you'd like to see an article on this subject in a future column?
Mike Talon
Where did this come from? Certainly not benchmarks performed on modern systems! Read performance is usually better since the there are multiple drives servicing the read requests. And, write performance hardly suffers at all.
It is very easy to demonstrate that RAID 1 (mirroring) has almost no write performance impact on hardware mirroring implementations (typically 1% or less), and very little impact on software mirroring implementations (typically 3% or less). I would be happy to provide people with a simple tool to prove this for themselves on their own systems.
The biggest problem we have ever seen is on older or low-end systems that have very small caching controllers, or that use software mirroring on underpowered or overburdened processors. Those systems typically have performance issues to begin with, so any configuration change must be done with care.
RAID 5 typically has a 50%-100% performance degradation on writes (even when all disks are working), and is far from foolproof. I have seen at least 3 systems (different clients) that had RAID 5 failures that required a restore from tape (controllers, more than 1 disk, driver failure). RAID 5 seems to give people a false sense of security.
Anyway, emailme if you want the simple benchmark program (free). We have a white paper that discusses RAID for those interested (www.comp-soln.com/whitepapers/disk_configuration.pdf) - it provides a simple, common sense approach to disk configuration.
Chip Nickolett (ChipN@Comp-Soln.com)
by miketalonnyc
Hi:
While I agree RAID 5 is not foolproof (note I mention that other DR options be employed), it does offer advantages over not having any redundancy at all.
As for RAID 1 (mirroring) not causing slow-downs on heavily utilized systems, the controller must make two writes for every write processed, and two reads in most implementations. That's twice the I/O of other systems (though some create even more I/O overhead) and therefore less performance than a non-RAID system. Even Microsoft reccomends that heavily hammered areas of disk used by their most top-heavy software (i.e. Exchange 2000) be RAID 5, and NOT mirrored. The system info is on mirrored disks, but the logs are reccomended to exist on other RAID configured partitions.Mike Talon
Hi Mike,
Thank you for taking the time to respond.
Yes, there must be 2 writes, but only 1 write needs to occur for the program making the call to continue (providing it even waits for the physical sync). Different implementations of RAID 1 handle read requests in different manners. Years ago (mid-80's with VAX disk shadowing) the algorithm for reads was not the most efficient, but on most platforms you will see performance that equals or exceeds that of a single disk (i.e., non-RAID).
I have sent our benchmark to several people based on this thread. Maybe a few of them will weigh-in on their test results.
There is always disagreement when discussing RAID, which is why we run benchmarks and get hard data. That is the only way to optimize performance and understand the limits of a high-performance system.
Chip Nickolett
Somebody who read this thread found that the URL for the white paper listed in the original reply is incorrect. The correct URL for the free disk configuration white paper is:
www.comp-soln.com/disk_configuration.pdf
I apologize for any inconvenience that this may have caused.
by david o
by miketalonnyc
The loss of a data-system is a disaster, regardless of what caused the loss. If the E-Mail server is taken out of commission, and E-Mail is critical to your organization, you have suffered a disaster.
RAID can circumvent the need to restore fromtape in many cases, and can help stop the need to fail over to another system or data-center in the event of a single data-system failure.
Mike Talon
by nbp backup
by miketalonnyc
I have personally worked for many companies that were too small to even consider affording anything beyond simple tape backup (which I mention is still required in my article). However, RAID was a methodology I was able to implement in order to save them from having to always restore from tape in the event of a single physical disk failure.
Tape is still required, but restoration times can be dramatically reduced by implementing RAID systems to handle minor disasters.
Mike Talon
by miketalonnyc
In almost all Windows systems, the write must be performed on the secondary volume before it is written to the primary, generally known as two-phase commit. This is what slows things down.
True, on other Operating systems this can be eliminated.
Mike Talon
by cglrcng9
...with a motherboard that came w/ onboard Promise RAID function and also purchased 3 identical same size Maxtor HD's, setting it up in 2 disk RAID 1 mirror, swapping out 1 drive (drive #2 on the mirror) after synchronizing a backup (which I store @the office in a carry box), each week I just shut off HD#2, hot swap and click synchronize and continue on working. I have the problem of disk failure prob's minimized, loss of data/time minimized greatly, and I have @ least 1 to 2 backup's handy most often.
I did however find the article interesting, articulate, simple and informative concerning the RAID function. It was also nice to know I wasn't alone in my thinking of using it as a backup method.
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