Understanding "RAID"

3 replies [Last post]
Joined: 12/06/2008
Posts: 7

Just some quick questions while searching the webz and watching videos, trying to understand how RAID works.

1. If you buy 2 SSD's with 1 TB each BUT setting them to RAID 1, will cause them to have 1 TB in total? yes, true? (Because 1 TB is actually used for backup)
2. What about in RAID 0? Yes you will have the total amount right? (2 TB)
3. If your two HDD's are running RAID 1, you can actually shut down your PC, tear out the other HDD and start it up again without it notice anything because of backup (mirror)?
4. Is RAID 1 faster, than using just a normal HDD (no raid) ?

Joined: 10/14/2010
Posts: 262

1. Yes, RAID 1 is mirroring. 1 TB total in your example.
2. Also correct. RAID 0 is combining.
3. Not sure if it would boot like normal, but in theory that should be fine. It should warn you that a drive has failed and you can add a new drive to rebuild the RAID. The part I'm not sure of is if it would allow you to continue since the RAID is broken. I guess that depends on the software, in any case no data is lost.
4. RAID 1 does not improve speed. Both drives are just writing the same data simultaneously. It's like two people writing the same essay and in the end you have two essays. RAID 0 is like two people writing one essay together, and in the end you have one essay done much quicker.

I think you're understanding it pretty well :).

Joined: 04/05/2010
Posts: 236

doesn't raid 1 increase the read speed because it is mirrored and can be read at the same time but different parts.

eire1274's picture
Joined: 09/12/2003
Posts: 1324

RAID0 can increase read speed because data is written in alternation across the stripe: 1 2 1 2 1 2 1 2 1 2

That will get you past head lag and push you closer to burst read speed.

RAID1 is actually slightly slower with write data because it has to be sent twice (once to each drive), and only the primary drive is read, so it functions as a single drive there.

RAID5/6 gives you multi-drive reading like a stripe (RAID0), so it is faster there, but is a little slower on writing as data has to be moved between the data and parity drives multiple times.

FlexRAID and RAIDZ or ZFS RAID is a simple software method for serving that does the above RAID methods in software. What's nice is that ZFS can have drives changed while the server is up, like DROBO. FlexRAID is still in development for Windows and Linux, and is pretty much a crash fest, but ZFS (formerly a SunOS component) is now part of FreeBSD and is available in the FreeNas distros. I use that in house as a data point for my video production in RAID5 on external drives.

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