Enable write back cache on RAID 5 good idea or bad idea

Posted on 2011-09-12
Last Modified: 2012-06-21
I just installed Intel Rapid Storage Technology and can now see what the heck I have been missing all this time. Wow.  Server has been running with no way to monitor the condition of the drives.  Now that I have this Intel utility running, the first question that comes to mind, if I enable the volume write back cache, is that good, bad? What is typically done?  This server has been running with that off, so I am thinking it will improve our wait time on SQL queries?  It is connected to a UPS with about a 2 hour run time, so I don't think power interruption is an issue. Any other considerations before I decide to turn it on?
Question by:rodynetwork
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 2
  • 2
  • 2
  • +1
LVL 47

Accepted Solution

David earned 200 total points
ID: 36526544
don't ennable it on that lowend raid.  too risky for data integrity, it has nothing to do with a ups. issue is how it handles timeouts and retries.
LVL 10

Assisted Solution

by:Mohammed Rahman
Mohammed Rahman earned 200 total points
ID: 36526669
Can have a look at the article below. It explains what exactly the RAID CACHING is. Also, it highlights pros and cons of it.

Author Comment

ID: 36527042
Our server supports 4 people on a LAN. We use Exchange for email and our desktops use an app that is interfaced via Internet Explorer and it uses SQL database queries all day.  I am wondering what part of cache enabling might cause data loss/corruption. Is it simply the functioning of the RAID, or is it the new data being written, or is it the read only that causes a problem? Is it possible to only enable read cache and not write cache (is that even feasible?) maybe that would eliminate the risk?  Our biggest time lag right now is simply having to wait on SQL queries to make certain pages display all the already written/stored data. If we could turn on the read aspect of the cache, maybe that would speed things up?
Instantly Create Instructional Tutorials

Contextual Guidance at the moment of need helps your employees adopt to new software or processes instantly. Boost knowledge retention and employee engagement step-by-step with one easy solution.

LVL 10

Assisted Solution

by:Mohammed Rahman
Mohammed Rahman earned 200 total points
ID: 36527156
Write-back Cache:
As per the article, whenever the data is recieved from X source to be written on RAID, the data is written on the cache and not on the actual disk. The cache then sends acknowledgement to the X source that the data was written on the disk. However, the data is still on the cache and not on the disk. Cache will take its sweet time to write the data to the disk (as per my inderstanding from the article).

The time required to flush the data from the cache to the actual disk may depend on the amount of data to be flushed onto the disk and the controller (hardware) used. Better the hardware, better the performance.

W have a backup of 2 hours through USP. All we should calculate is the following below:

Amount of data that the cache can handle. (can write on itself before flushing it to he actual disk)
Time taken by the controller to flush entire data from cache to the disk.

If the time taken above exceeds the 2 hours, we may either plan to drop the write-back cache OR increase the backup time.

Write-Thru Mode.

This mode does not utilize the RAID cache for accelerating write I/O requests. In most cases it will be slower than Write-Back mode.
Hence, selecting Write-back over Write-thru will be a good deal (provided, we have a proper electricity backup).

I am wondering what part of cache enabling might cause data loss/corruption? (Looks like the Write Back will result in data loss and crruption in the event of power failure if the time taken by the cache to flush the data to the disk is more than 2 hours)

You can also read write-back vs write-thru and the conclusion from the article below:

** After reading all these articles, I would suggest not to turn it ON unless you are expecting drastic improvement in performance. Or, check if we can get any software to benchmark both and take a call.
LVL 47

Assisted Solution

David earned 200 total points
ID: 36527346
First, you could have another issue .. specifically what HDDs do you have?   But ignoring that for a moment, the RAID controller you have has issues with RAID5 and it is just a low-end controller and simply not suited for multiple error scenarios.   While the write-back/write-through information above is correct, the information does not take into consideration error recovery scenarios when you do not have a battery protected controller.

write back cache can result in data integrity/bit rot issues that make it  unsuitable for RAID5.
LVL 69

Assisted Solution

Callandor earned 100 total points
ID: 36528991
Like the others, I would advise against using a write back cache if you are not using a hardware RAID controller with battery backup.  The possibility of losing power, even with a UPS, means any data in the cache not yet written will corrupt your database, so I would urge using the most stringent precautions.  You can use a read cache with no problems, and it should be possible to enable it separately.

With only 4 users, your database query problems are probably not due to the hardware, but to the design of the queries.  You might want to ask an SQL question to get the experts their take on it and see if you can optimize them.

Author Closing Comment

ID: 36529424
Good input guys. Thanks for keeping me out of self inflicted trouble!

Featured Post

Enroll in June's Course of the Month

June's Course of the Month is now available! Every 10 seconds, a consumer gets hit with ransomware. Refresh your knowledge of ransomware best practices by enrolling in this month's complimentary course for Premium Members, Team Accounts, and Qualified Experts.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

this article is a guided solution for most of the common server issues in server hardware tasks we are facing in our routine job works. the topics in the following article covered are, 1) dell hardware raidlevel (Perc) 2) adding HDD 3) how t…
Great sound, comfort and fit, excellent build quality, versatility, compatibility. These are just some of the many reasons for choosing a headset from Sennheiser.
If you’ve ever visited a web page and noticed a cool font that you really liked the look of, but couldn’t figure out which font it was so that you could use it for your own work, then this video is for you! In this Micro Tutorial, you'll learn yo…
This is my first video review of Microsoft Bookings, I will be doing a part two with a bit more information, but wanted to get this out to you folks.

705 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question