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RAID is useless
I know RAID is not for backup purposes
stidj




msg:1566639
 5:09 pm on Aug 2, 2005 (gmt 0)

Hi guys

I used to think RAID-1 was helpful for redundancy to keep data intact and accessible if for example 1 out of 2 drives fails.

I used to think it is as simple as if a drive fails the other one works perfectly since that data is preserved and not corrupt.

What I don't understand is the point of it anymore for a number of reasons. I understand that if one drive has bad sectors then the corrupt data will simply be mirrored to the other one (until the drive dies) so at best you may have a server up that continues to operate in a useless way since it will have corrupt data.

Am I missing something here or is there a way to make better use of RAID?

If I am right it seems RAID-1 is at least is hardly useful in keeping data accessible at all times since it can't protect against data corruption from another drive.

Please explain in detail about if I am misunderstanding.

 

oddsod




msg:1566640
 5:20 pm on Aug 2, 2005 (gmt 0)

There are several reasons why RAID is not a good idea if you're not a typical transistor head. This article [bestpricecomputers.co.uk] covers the point you make about the second drive being a backup of the first, warts and all. There are some tips on protecting yourself if you are currently using RAID... but the tips are not much use if you've already run into problems.

RAID is not a substitute for good backing up procedure and practise.

stidj




msg:1566641
 5:30 pm on Aug 2, 2005 (gmt 0)

Thanks for the article but I am already aware it is not a backup solution and it doesn't seem to address any of my points :(

How do the big guys ensure disk failures have no negative impacts (that all data is always accessible without corruption due to a failed drive in the array)? I guess maybe they use a huge cluster of RAID 5 drives so that the parity allows them to recreate any corrupt data?

Is there another solution in a single system to remove/reduce the risk of data corruption with RAID-1?

oddsod




msg:1566642
 5:36 pm on Aug 2, 2005 (gmt 0)

>> Am I missing something here or is there a way to make better use of RAID?
RAID 1 protects against a disk breaking down, that's all. Different other versions perform different functions, of course.

>> huge cluster of RAID 5 drives
Some do, but only very big outfits. Most rely on a good backup system. Some of the enterprise grade backup products are phenomenal.

>>I understand that if one drive has bad sectors then the corrupt data will simply be mirrored to the other one
That's not how it's supposed to work. When it develops bad sectors it warns you and you then call upon your redundancy to replace the faulty drive.

>> Is there another solution in a single system to remove/reduce the risk of data corruption with RAID-1?
There's the less talked about Matrix RAID - provided you have the right hardware - but, I'm afraid, it's probably not what you're looking for. You could have a third hard disk and regularly Ghost/DriveImage to this drive either manually or automatically.

MattyMoose




msg:1566643
 5:47 pm on Aug 2, 2005 (gmt 0)

**
I'm assuming that you mean "corrupt data" because of a bad disk (you mentioned bad sectors), and not corrupted data due to a faulty application.
**

Sometimes yes, sometimes no, depending on the implementation. There are two ways to implement RAID: software and hardware.

The "better" and more expensive way is usually through an IDE/SATA/SCSI controller that has a microprocessor on it that will handle all the functionality invisibly. This means it will do checksums on written data, and allocate the disks in the "best" way for you and so on. If you're using RAID on production/critical data, use this... You probably already know all that though.

Anyway, HW RAID is typically more reliable than OS-level software RAID, and in this scenario that you mention, I don't believe that it will duplicate corrupt data, EXCEPT in a scenario where one of the disks dies, and you rebuild your array using the corrupt disk. Then you'll get bad data being mirrored to your new drive.

Typically though, you won't encounter this scenario because most disks have built-in checksums, the RAID card itself will validate written data with checksums/hashes, as well as OS-level verifications of written data.

Software RAID takes data that would have been written to one disk and simply mirrors the same write functions to a second disk. The same reasoning applies as above as to why bad data might appear, with one added scenario:

The RAID implementation writes to a corrupted disk FIRST, then reads the data back from the corrupted disk, and writes corrupted data onto the second disk.
(make sense? Write to disk0, then read from disk0, then write to disk1)

This would be a piss-poor RAID implementation, IMO, but it may exist since it may speed up the initial write operations, so rather than way for two disks to write, the hook into the disk writing functions from the application is freed faster by only writing to the one, and the RAID software is called in the background to mirror the changes. See what I mean?

Either way, I don't believe that the scenario you're describing is all that likely, considering all of the checks that are in place.

my 0.02

stidj




msg:1566644
 5:54 pm on Aug 2, 2005 (gmt 0)

To clarify, I'm talking about corrupt data relating to hardware failure/error

Thanks oddsod, so if hardware RAID is being used and a drive dies or one drive is corrupt how does it warn me or does it just stop using that drive in the array?

MattyMoose thanks for your help. So basically what you're saying is that in a hardware RAID situation it is nearly impossible to have corrupt data relating to physical errors mirrored to a good disk?

I suppose when you talk about the poor implementation of RAID-1 that you are referring to possibly software RAID where things could more likely go wrong (eg. writing corrupt data to another drive)?

Thanks again guys hopefully I can get this figured out :)

aspdaddy




msg:1566645
 10:05 pm on Aug 2, 2005 (gmt 0)

> so if hardware RAID is being used and a drive dies or one drive is corrupt how does it warn me or does it just stop using that drive in the array?

My raid 5 array was tested like this - grab any drive and just yank it out the server. The lights come on, the other drives take straight over without any problem. Now get a brand new drive out the box and just shove it in, it builds a copy of the data in less than a minute and you are back to normal.

Thats what it does, it protects from downtime and corruption when a drive fails and needs changing.

I think monitoring your drives is a different issue or maybe some RAID software can send a warning to you?

oddsod




msg:1566646
 10:15 pm on Aug 2, 2005 (gmt 0)

>> grab any drive and just yank it out the server

Please do that only if it's hot-swappable ;)

MattyMoose




msg:1566647
 4:57 pm on Aug 3, 2005 (gmt 0)

So basically what you're saying is that in a hardware RAID situation it is nearly impossible to have corrupt data relating to physical errors mirrored to a good disk?

Yep, exactly that. All of the RAID hardware manufacturers take their job seriously, since you just paid a lot of money for something that needs to be flawless. ;) Possible exception being possibly consumer-grade RAID devices like on-board Promise RAID controllers.

I suppose when you talk about the poor implementation of RAID-1 that you are referring to possibly software RAID where things could more likely go wrong (eg. writing corrupt data to another drive)?

Usually, yes, but I don't really know of any RAID software or hardware that is of such bad quality to do something like that. It wouldn't take long before someone realized this and boycotted the product in question. ;)

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