RAID Idea

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ok guys, heres a question for you. say i want to use RAID 0 on my pc in order to speed up data access. now i see a problem here (and know of someone who had it): if one of the two drives fail, pfft - end of data. its impossible to recover the data since you only have half of each file, essentially. so - what if you connected a 3rd drive, using RAID 1? (see attachment), this would allow increased access time, and provide a data backup.

is this possible? and what about RAID 5? it provides a parity code, but surely if one of the drives crashes, end of data anyway????? or am i misunderstanding RAID 5?

also - if i use RAID 0 with 2 80GB drives, i get 160GB's right? sounds like a daft question, but i just wanna be sure.

and finally, use raid like the example above - how many drives does my system see??????

Fergal

 

ok, so according to that link, my idea is pretty much the same as RAID 5 correct? with 3 80GB drives i would have a total of 160GB's of data storage space.

can someone explain to me how the parity system means you dont effectively lose the data if one of the drives fails? i dont really understand that.

Fergal

 

To quote from the Exam Prep text Server+ by Bird and Harwood:

"If one (of the three) disks should fail, the other disks would continue to function without it. This is possible because the system can calculate the data that was on the missing drive from the parity information. Using the first stripe as an example, knowing that the data from the first disk is 3 and that the parity is 5, the system can deduce that the missing data is 2 (parity of 5-3=2)".

And that's the simple explanation. Basically, in addition to each drive storing a portion of the overall data, it also stores parity information that any two of the disks can use to calculate the data that is on the third disk. Of course, this means that you have to have the disk space not only for your actual data but for the parity data as well. In a five drive system (according to Bird and Harwood), 20% of your disk space is used for parity. In a four drive system, 25% and so on. On the other hand, if one of your drives goes toes up before you've done your nightly backup, your data hasn't disappeared forever.

While writing this, I tried to find a detailed explanation of RAID 5 on the web but thus far, I've been unsuccessful. Hope what little I've been able to bring to the table explains it just a bit for you.

 

a brief overvue of Raid

Raid 0, Striping, its called Raid 0 because it offers 0 redundancy
Raid 1, Mirroring, it mirrors one disk to another
Raid 2, Duplexing, same as raid 1 but it uses seperate controllers also
..
Raid 5, Striping with Parity
Raid 5e, Strping with Parity + Hot Swap Spare

your doc wouldnt do much, any RAID level requires a minimum of 2 disks
Raid 5 requires 3
Raid 5e requires 4


Raid 0 offers the best performance (increased based on the number of disks you use) but it offers no redundandy, infact it hampers redundancy as one failure = total data loss, wheras JBOD would mean all remaining disks are uneffected

Raid 1 is rather wasteful as you will only ever get 1/2 the space you put into an array thats usable, but you retain a copy of data without a rebuild process being required, its available on most ide raid controllers though

Raid 2 is much the same as Raid 1 with an added layer of redundancy

Raid 5 i think is what your looking for
you however need 3 disks (one will be wasted) Raid 5s waste is decreased with numbers as you only ever use ONE disks capacity for parity, so the more disks, the better. speed also increases with number of discs(same with raid0)
and raid 5 generally requires a special controllers, most IDE raid controllers dont offer that usually

Im personally building a system involving Raid 0 and Raid 5 at the moment
I will be using a 3Ware 8500-12 SATA Raid controller (about $700 USD)
9x 200GB SATA drives in Raid 5 (Data) (Capacity = 1.6TB) ($130 each)
3x 74GB Raptor 10K SATA Drives (Apps/OS) (Capacity = 222GB) ($200 each)

any more Qs just ask

 

Just to clarify
like Trip said the capacity of a Raid 5 System should be a percentage not 'a drive' however it is advised that you use identical size drives (in an idea situation you should use identical size/model/even matching serial number drives) however that kind of knit pickyness has been ground out of most new controllers, but as its best practice to use drives of the same size, you effectivley loose one drive out of your array, thats all

 

ok, thats cool, i was intending to connect the HDD's through SCSI to be honest. RAID 5 definately sounds like my best option, since is offers increased speed, AND redundancy.

if one of the drives were to fail, and i replaced it with another - how would the system deal with it? would it just start using it? or would i need to have the system 'rebuild' the data onto the HDD?

just to clarify - if i were to want to add additional drives to the system using RAID 5 - is that possible?

and finally - does that link mean that i can only use RAID 5 if i have 2000server? what about if i want to use xp on a home pc - would pro allow this? or would home?

Fergal

 

not quite sure what your budget is but a SCSI raid 5 system dont come cheap


short answer is depends on the controller
some controllers will auto rebuild an array when a new drive is added, others need to have it kicked off manualy via the array config tools

Raid 5 E uses a hot swap spare so that if one drive failes it automatically rebuilds the array to the spare disk without user intervention (you would still need to actually replace the disk in a standard raid 5 system)

 

gash - just had a look, and a SCSI RAID 5 capable card is like 300 quid. might just have to wait a while for that one, depending on whether i spend a fortune on the other stuff to be honest. damn!!!

never mind i spose - good to know anyway.

Fergal

 

trust me mate 300 quid for scsi raid is pretty darn cheap, a decent controller runs at 3000 quid+
and 146GB U320 10k disks run at 800 a pop
compared to 250GB 7200 SATA disks running at 150 a pop

 

SCSI stuff is always more expensive

 

You can set up RAID as a software solution in Windows 2000 Server or Windows Server 2003. Still going to have to spend some money on the deal but as Sandy said; SCSI/hardware solutions are always more expensive. The other option is to "rescue" an older server from some local business. Many business decomission their older servers (running NT) because both the hardware and OS are obsolete. After they wipe the hard drives of all the data, they either donate the equipment to schools or charitable organizations or sell their hardware for a very inexpensive price.

Last weekend, I was on a team that did a hardware and operating system upgrade which included decomissioning several servers. All the equipment was destined to be shipped back to the home office. If you could figure out who is getting rid of old equipment, you might be able to pick up a RAID capable server cheap.

One of my former instructors was an expert at rescuing old equipment. Sure, it would only run Windows NT but you could still play with RAID configurations and get a feel for how they work. Just a thought.