Why RAID rebuilds obstruct data migration

Why RAID rebuilds obstruct data migration

I currently run a system using four 400 GB SATA drives configured with RAID 5. I want to replace all drives with four 750 GB drives. Can I do this by swapping individual drives after each is rebuilt? The SATA controller is the Nvidia MediaShield, and supports RAID 0, 1, 0+1, 5 and JBOD.

    Requires Free Membership to View

    Login

    When you register for SearchStorage.com, you’ll also receive targeted emails from my team of award-winning editorial writers. Our goal is to keep you informed on the hottest topics, the latest news and the biggest challenges you face as a storage professional today.

    Rich Castagna, Editorial Director

    By submitting your registration information to SearchStorage.com you agree to receive email communications from TechTarget and TechTarget partners. We encourage you to read our Privacy Policy which contains important disclosures about how we collect and use your registration and other information. If you reside outside of the United States, by submitting this registration information you consent to having your personal data transferred to and processed in the United States. Your use of SearchStorage.com is governed by our Terms of Use. You may contact us at webmaster@TechTarget.com.

Tragically, your proposed method is not the best way to migrate your data. Remember, RAID rebuilds take quite a long time and are a resource-intensive operation. So expect your application to nicely suffocate while this is happening.

RAID rebuilds are a protection mechanism and are not designed for data migration. To this point, assuming your 750 GB drives are supported with your RAID adapter, the RAID group will only utilize the first 400 GB of storage on each 750 GB drive.

Remember, RAID groups typically require that all the drives in the group are of the same type, speed and size (see my previous articles for more details on this). This means that even after you go through your "migration," you'll still only have four 400 GB SATA drives worth of available storage. The remainder will be inaccessible.

The usual way to migrate data with internal server-based storage is to copy the data to another separate set of drives or temporary storage, internally or on your network or a (wince) large USB hard disk drive (the wince means "not recommended but can suffice if you're in a pinch"), install the new drives and copy everything back, remembering to preserve all file attributes such as permissions and access dates, etc. You can use a tool such as Robocopy.

It's headaches like these where a consolidated mid-tier storage solution, such as those from EMC, IBM and HP, comes in handy. They can migrate data from one RAID group to another without any disruption and usually while the application is still accessing the volume! If your company can afford it and has a significant enough storage management requirement, I strongly suggest investigating this approach.

This was first published in January 2008