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adad
20th July 2006, 02:22 PM
I've been googling about this concept, but only got ambiguous documents as a result.

What I'd like to know if there's a way of building a RAID array with partitions, not whole disks. An example: you have a single 300 GB disk, partitioned in 3 x 100 GB partitions. Using such partitions you build a raid 5 structure.

I know I would kill the system performance, and it would be veeery slow, etc., but you would get a disk with redundancy. Is that possible with LVM, software RAID or any other method?

ccrvic
20th July 2006, 02:27 PM
Is that possible with LVM, software RAID or any other method?

Yes.

man mdadm

Vic.

adad
20th July 2006, 08:52 PM
Thank you for your answer. Does mdadm allow for RAID 5 say, in 3 partitions of the SAME disk?

pjfg
20th July 2006, 09:19 PM
I'm sure it would but the question is why? Performance would be horrible.

Anyhow, as to how it normally performs - I've only used linux software raid on scsi (and then only mirroring) but performance seemed comparable to hardware raid.

adad
20th July 2006, 10:05 PM
It's not a question of performance. I was wondering if there was a way of having a slow but reliable backup system, just for backing up incremental backups for my home office, where only a few files are modified on a daily basis, but these changes can't run the risk to be lost. I was thinking about having redundancy on a small system, e.g. no more than two disks. I'm after a small and quiet equipment: no extra big power source, only one cooler for the cpu, etc. And maybe, if at all possible, everything in a single 300-400GB disk.

pjfg
20th July 2006, 10:44 PM
What you were proposing ( 3 partitions on the same disk ) wouldn't be a recommeded "backup" solution anyway, since when the disk fails you'll lose everything anyway. What I do for home is plug in a large external USB drive and have a cron job schedule an rsync of my data overnight. Perhaps that is a more suitable solution for your needs.

the poi
20th July 2006, 11:13 PM
What you were proposing ( 3 partitions on the same disk ) wouldn't be a recommeded "backup" solution anyway, since when the disk fails you'll lose everything anyway.
ya, that would be a terrible idea!

pparks1
21st July 2006, 12:29 AM
Yes, this is NOT fault tolerance at all. You won't have a partition fail, you will have a hard drive that fails. When that happens, you are going to to lose all of your partitions, thus the RAID configuration got you absolutely nowhere.

Your options for a backup include

#1. Install 2 discs and configure them in a RAID1 (mirror). This would give you the exact same thing on 2 drives, thus if 1 drive failed, you would still have the other drive and would lose nothing. I use this at home

#2. A backup routine (I use rsnapshot from www.rsnapshot.org) to do a recurring rsync backup to another host on your network. The best part here is that the data is in two locations. Heaven forbid the 1st server catches fire or is stolen, but if either happens, even a RAID 1 (mirror) cannot save you.

#3. An external hard drive. This is a great solution because you can easily keep it off site. I also use this at home.

So, if you read correctly, I use all 3 methods to protect my home data. I use a RAID1 mirror, i use rsync to another host on the network and finally I use a portable USB hard drive which I keep offsite. It's a bit overkill, but I don' t ever lose anything important.

rclark
21st July 2006, 12:51 AM
Agree. Not a good idea at all. It's always a disk failure that gets you.

As pparks, at home I always backup to another system which was an XP box, but now backup is a 300GB USB external drive. Also I save the most critical data on DVDs as well (like pictures, financial data, etc) as this data will all fit on CDs or DVDs. Operating systems can always be rebuilt from install disks, but not your data. Protect it well if you can (or care)! Note I don't use a RAID at home, as I feel I can stand some down time if necessary to build up another system. As long as the data is 'somewhere' I am good to go!

At work we use disk mirroring as downtime is a bit more critical here. Drives are cheap and big, so I don't mind using RAID 1 for my OS system hard drive and also my data hard drives. I like the simplicity of simple mirrored drives for a file server. We also backup to tape for the 'real' backup (full and incremental) and keep the tapes in another building (off site). Remember, while another disk saves your 'current' data as is, it doesn't prevent you from accidently deleting your critical files!!!! That's where the backup to another system, cd, dvd, tape comes in! Just a heads up to those who might think a RAID system is good enough!

pparks1
21st July 2006, 02:47 AM
it doesn't prevent you from accidently deleting your critical files!!!! That's where the backup to another system, cd, dvd, tape comes in! Just a heads up to those who might think a RAID system is good enough!
Very good point and one that I should have made. RAID also will NOT be able to protect you from a virus either. It will wipe out both drives very quickly. You absolutely must have a backup of your critical data. RAID provides fault tolerance....not a backup

ccrvic
21st July 2006, 10:15 AM
Yes, this is NOT fault tolerance at all. You won't have a partition fail, you will have a hard drive that fails.

That's not *strictly* true; it is entirely possible that the drive failure will be a degradation of the magnetic surface (particularly if you've bought a Maxtor in the last few years...). This could give you parts of the disk that are entirely inaccessible, while other bits read fine (and that ~1Hz tick that always accompanies it).

However, the risk of a total disk failure is very real. The performance of a RAID-5 on a single drive would be appalling. Overall, this is an incredibly daft idea...

Vic.

pparks1
21st July 2006, 03:26 PM
particularly if you've bought a Maxtor in the last few years...
Yes, my experience in the last year or so has been exactly the same with regards to Maxtor. We've had a ton of Dell workstatations at work just going through the Maxtor drives like crazy. We had Dell send us Seagate replacements and everything has been great. And these days, Seagate owns Maxtor....hmmmmmmm