• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 311
  • Last Modified:

Raid 1 and Raid 5: Example of a problem that will make one crash and not the other.

Hi.  I believe I understand how space and cost is divided between Raid 1 and Raid 5.  A lot of members here say that Raid 5 is the best way to go.  I can't afford to combine a Raid 1 with a Raid 5, so I have to choose.

My question is:

What can happen with a Raid 1 system that will not happen with a Raid 5 system.  Can someone tell me an example of what would make a Raid 1 system crash and would not make a Raid 5 system crash.  For example, if I have a bad sector on a Raid 1 system, it will be mirrored on the second drive.  If I have a bad sector on a Raid 5 system, what will happen?
0
TIC_Telecom
Asked:
TIC_Telecom
3 Solutions
 
tcicatelliCommented:
Raid 1 is fault tolerant mirroring.  You get a small performance hit when you write, but gain when you read as data comes from both drives.

Raid 5 has a similar performance hit on write, but a much better gain with read speed as all drives feed data at the same time.

Bottom line: Both systems have similar fault tolerance; if you have a performance problem with many people reading from the fileserver at the same time, then raid 5 would diminish this bottleneck.

I dropped from Raid5 to Raid1 utilizing 2 WD 10,000 RPM Raptors and have never looked back.

-Tom
0
 
cuziyqCommented:
As far as fault tolerance is concerned, both a RAID 1 and a RAID 5 will give you the same benefits.  If you lose one of the drives, you can replace it and rebuild (neither can withstand the loss of more than one drive, however).

The advantage of a RAID 5 volume is the amount of space consumed for the failt tolerance.  In a RAID 1 setup, you will have exactly half the capacity of the sum of all the drives in the array.  So if you have 4 drives, you lose 2 of them for fault tolerance.  In a RAID 5 setup, you only lose the capacity of 1 drive no matter how many there are.

RAID 1's have the big advantage in performance, however.  If a drive should fail in a RAID 5 setup, access to the data in fault mode is really really slow until the replacement drive has been rebuilt (which can take hours).  RAID 1 suffers no degredation in performance in fault mode whatsoever.  Also, a RAID 1 is faster at writes than RAID 5 (although RAID 5 is faster at reads).

So to answer your question, both RAID setups will give you the exact same level of protection.  There is nothing that can happen to one RAID setup that cannot happen to the other.
0
 
cuziyqCommented:
Tom's post reminds me something about RAID 5 setups that makes me scratch my head a little.  You MUST have at least 3 disks to have a RAID 5 setup, while a RAID 1 setup only requires 2.  Tom . . . how did you convert two WD Raptors from RAID 5 to RAID 1??
0
Concerto's Cloud Advisory Services

Want to avoid the missteps to gaining all the benefits of the cloud? Learn more about the different assessment options from our Cloud Advisory team.

 
tcicatelliCommented:
I didn't convert raid 5 to raid 1; when I swapped servers, I left an old dual P3-350 server with raid 5 (and the associated overhead of 5 failing hard drives) to the simplicity of a dual - mirrored P4-2400 machine.  In either case, the O/S only sees one logical drive.  I managed to reduce heat, maintain performance, lower acquisition costs and I haven't been happier.  It's really a read performance issue, and you're going to pay in $ and maintenance.  IMHO, I see a 5 or 6 drive Raid 5 system as 5-6 drives waiting to fail.

-Tom
0
 
tcicatelliCommented:
I think Raid 5 had more of a place when we were looking for more storage space, the drives were slower, and we wanted fault tolerance.  Raid 1 has caught up in that we can buy a single drive that has all the capacity we need, and the speed of these drives is incredible.  
0
 
andyalderSaggar makers bottom knockerCommented:
You might be able to combine RAID 1 with RAID 5 for less than you think; some controllers allow you to put all the disks in a single bunch and then greate several arrays on them so 4 disks could have a RAID 10 slice for boot, a RAID 5 slice for slow data, and a RAID 6 slice for even slower data. Whether that is advisable or not depends what the server is being used for.

>Can someone tell me an example of what would make a Raid 1 system crash and would not make a Raid 5 system crash.

Take a two disk setup: power it off, remove a single disk and replace it with one from an old server and power it on again. The controller now has disks from two different systems and doesn't know which one has valid data on it so the data from the old server might get written onto your good disk. It's much harder for this to happen with a system of three or more disks. The problem is avoided with hot-swap disks by not powering it off when you swap disks and generally the controller complains during boot anyway.

Bad sectors don't normally come into it since the drive will map them out and either RAID level will cope with 1 disk failing.

RAID 5 has a high write penalty, 4 physical I/Os for each logical write, RAID 1 or 10 only has two physical I/Os per logical write. Therefore during write a 4 disk RAID 5 system is fractionally slower than a single disk excluding caching on the RAID controller.

Echoing tcicatelli I don't see RAID 5 as valid in a business environment any more with the enormous sizes available anymore, if you need performance just use RAID 10.
0
 
TIC_TelecomAuthor Commented:
Thank you all very much for the quick answers.
0

Featured Post

Concerto Cloud for Software Providers & ISVs

Can Concerto Cloud Services help you focus on evolving your application offerings, while delivering the best cloud experience to your customers? From DevOps to revenue models and customer support, the answer is yes!

Learn how Concerto can help you.

Tackle projects and never again get stuck behind a technical roadblock.
Join Now