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multyhdds/raid what is the difference?

Posted on 1999-01-04
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What is the difference between setting up 4 10G HDDs and a RAID that is made of 4 10G HDDs?  A RAID setup only uses 1 interface right? Is there a way to set up more than 1 HDD on one interface?  Possibly a devise that could be bought that has all the parts of RAID devise but you hook up the HDDs?  And does a RAID setup write the same data on each drive to get the better read speed or is it just that it can write different parts of the data at the same time to achive better write speed or both? And overall is it worth it? There's a lot so I plan to leave open for comment for a little while.
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Question by:jasonrun
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by:harrys
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There are serveral levels of Raid.
Raid 0 Stripping
The content of the device ist split across the disks in the array, this is faster than a single disk since the i/o operations are spread among the disks, but its not redundant. Its also unsafe to have a stripped array with lots of disks since the more disks you have, the more likely is a failure of a disk
Raid 1 Mirroring
Every disks has the same content, Raid 1 is redundant and in some implementations you gain some speed on reads, but you only have the capacity of a single disk
Raid 4 Stripping with parity disk
same as Raid 1 except of the parity disk, this disk is used to rebuild the data of the array in case of a failure of a single disk, Raid 3 is redundant and does fast reads, the drawbacks are the loss of capacity for the parity disk and the slower writes, because the parity must be written onto a single disk
Raid 5 Stripping with stripped parity
same as Raid4 except that the parity is spread across all disks. This is what most people mean, when the say Raid ...


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by:jasonrun
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thanks harrys,
If you could tell me about "building" one i'll give you the points.
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by:dpuckett
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If you set up 4 single drives then you would have 4 seperate volumes.. like c:,d:,e:,f: drives, where a raid array presents all drives as one volume, pehaps d:.

SEE harrys's comment for RAID 0 - 5 descriptions

When you speak of intefaces, I assume you are referring to the drive contoller, but you do not specify whether you are using SCSI, IDE, or extremely unlikely, MFM.  If you are using IDE, then you will probably be limited to 4 drives ( Master+Slave IDE0 + Master+Slave IDE1) on a controller, where if you are using SCSI then you could have 6 or more drivers depending on how many channels, scsi level, etc..

It is not always desirable to have all the drives you can fit on a controller, several drives can easily exceed the available bandwidth, and you will sacrifice performance.

There are SCSI contollers that are also RAID controllers, HP makes HPNetRaid, but It's not one I would recommend, unless you are happy with Narrow performance, The DPD SmartRaid4 is a product that I am also familiar with and am very happy with it.  It has 3 SCSI channels, 4-64 meg of onboard cache and rocks! <- a technical term used in reference to high levels of throughput :)

In choosing the RAID configuration, you must take into account several factors, speed over safety, space over speed, safety over space.

RAID-0: similar to a novell volume spread over multiple partitions, loose a disk, potential goodbye to all data data, but not bad performance, space is total space of all drives.

RAID-1: good speed on reads, poorer performance on writes, total space is equal to size of smallest drive in array, data is very safe (hell, it is written everywhere).

RAID-3: data is wriiten to number of disks less one, a parity value is wriiten to the last drive, so your space is (n-1)/n of your total space, the data is quite safe in that you can loose any one drive, reads are fast, writes are not as slow as RAID-1, but slower than RAID-0.

RAID-4: sort of combining RAID-0 with RAID-3

RAID-5: Parity striped accross all drives in array, data is stripped across all drives in array. Fast reads not bad writes, space is (n-1)/n of your total space. The parity value is written to the next drive in the sequence, you can loose any one drive and your data is safe., many RAID controllers also allow for a 'hot spare' drive.

I use a DPT SmartRaid4 controller, have 7 seagate 9 gig barracudas configured in a RAID5, so.. although the total space is 63 GIG, I have 54 usable GIGs of space, the data will be safe if I loose one drive, 3 drives are on one channel, 4 drives on another channel.

This configuration is on an NT box, and the OS 'sees' just one 54GIG volume. Hope this makes things a bit clearer for you.


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by:harrys
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How much money do you want to spent?
What operating system do you use?
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by:jasonrun
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Well i dont have much money right now. I use NT4.0. The money is the reason for the question.  I can get some maxter diamondmax 17G HDD for $338 each and was wondering if i could get them one at a time and eventually put them together in a way to make them act like a RAID setup. I dont have a SCSI controller yet but I wouldnt mind geting one. The maxters are ultra dma EIDE.
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by:heathprovost
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I will address your questions 1 at a time:

>>What is the difference between setting up 4 10G HDDs and a RAID that is made of 4 10G HDDs?

A. Alot. 4 10G drives are just that, 4 individual drives.  Assuming each drive only has 1 partition (NTFS or FAT32), you would end up with 4 individual drive letters.  A RAID setup with 4 10GB disks (assuming Level 5 RAID) would show up as 1 drive letter with a size of approximately 35 Gigs (dont remember formula for computing size - it is total size minus size of parity data, but it is close to 35). You lose a little size to the parity data.

>>RAID setup only uses 1 interface right?

A. Not necessarily, it can have more than one interface.  Also you can have only 1 interface when using 4 individual drives so there is really no difference as far as interface count goes.

>>Is there a way to set up more than 1 HDD on one interface?

Yes. On IDE you can have up to 2 drives per port.  On SCSI you  can have up to 6 (on some cards up to 14).

>>Possibly a devise that could be bought that has all the parts of RAID devise but you hook up the HDDs?  

A. Adaptec makes a SCSI RAID controller that can have up to 14 devices on it all part of the RAID array.

>>And does a RAID setup write the same data on each drive to get the better read speed or is it just that it can write different parts of the data at the same time to achive better write speed or both?

A. Both depending on RAID Level. See harrys comment.

>>And overall is it worth it?

A. Depends on what you need it for. Raid 0 is primarily a performance enhancement and can reduce I/O times on a very busy server tremendously. Raid 1 and 5 are primarily data recovery enhancements. For instance under Raid 1, if you lose a drive your server doesnt even go down. I/O request are routed to the other drive in the mirror. You can even do some interesting things like have 2 Raid 0 arrays with 4 drives each and then mirroring the arrays.  This is kind of the best of both worlds but is very expensive.

Hope my comments help.

 
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by:harrys
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Sorry, but I have never build an array using IDE drives..
I quite sure that the is no software Raid solution for NT (like for linux or freebsd), so you would need a hardware solution (like the DPT SmartRaid mentioned earlier in this thread), but for EIDE it will be difficult to find...
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by:Jason_S
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If you go into NT's Disk Administrator, you can setup your drives as you like with no additional software, or hardware.  You can either setup a Mirror set, Stripe set, or Volume set.  

Mirror set will make exact copies of your data across two drives.  This will only let you use half of your alloted drive space, but is most dependable.

Stripe set will use two or more drives, and set it up as one partition.  The drives, or at least free space used on each drive must be the same across all drives.  This is going to give you good data integrity.

A volume set will let you take all free unpartitioned space on all drives, and set it up as one partition.

Any questions, let us know.
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by:Jason_S
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Two close posts above, so I thought I would clarify.

Yes, NT's Disk Administrator will let you set this up on IDE drives.
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by:jasonrun
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Ok tha NT disk admin sounds VERY good price wise:) the DPT smartraid is out of my price range right now so I think I will just get the 17G maxter even though it only has a 5400 spindle speed. Does seting up the mirror add to the performance as well as security? I dont feel like going to SCSI right now and I have exactly 6,586,368 BYTES!! left on my only 2.1G HDD If anyone can tell me how/where I can get more them 17G for $338 or atleast 13G at a better quality for same or less price I will give them the points  otherwise I have no clue how I am going to distribute them
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by:Jason_S
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Mirroring wont increase performance at least not noticably.  RAID with IDE is more for data security, than performance.  I suggest using a Stripe set for best results.

Lots of good information here.  It's up to you for distibution of points.
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by:jasonrun
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25 for all?  how do I do this?

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by:Jason_S
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Normaly a question's points can only be paid out to one person.  To split up the points, you need to post a zero point question in the Customer Service area requesting this.  Then they will inform you of what to do.
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by:cpt823
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The difference between raid and stand alone hd's, is that when you use raid, u use the combined space of 2 hds, and have a 3rd for a CRC table.  That way, if one fails, you can re-construct the hard drive by sticking it in, and re-creating it from the crc table & the other hard disk.  The other method, if a hard disk dies, you are screwed.  You lost all of your data, if it can't be repaired.
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by:Jason_S
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cpt823:  I see that you are new here.  This is a great site, and you will learn allot from it.  Welcome.

Unles you have a definative answer for a user, please try to  add comments only until you are asked to submit an answer.  Answering a question will leave it locked to many users, and will diminish visibility of the question.  Thanks.
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by:dpuckett
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do ppl not read the existing comments before answering?
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by:jasonrun
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maybe you didnt hit the show all click here botton.  its been answered and i just am trying to figure out how to give four people the points
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by:bushhead
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u can actually have more than 4 IDE hdd's by getting an ide controller card which would take up one ISA or PCI slot on your motherboard.. i thot about doing this cos i ran out of  ide channels, but i also ran out of IRQ's... sucks..

bush
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by:jasonrun
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yeah i knew that and id like to get this question off.  if none of the four people I listed before wants the points or will tell me how to divide it between them then I will just delete the question in a couple days.
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by:Jason_S
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I did above.  here it is again.  : )

Normaly a question's points can only be paid out to one person.  To split up the points, you need to post a zero point question in the Customer Service area requesting this.  Then they will inform you of what to do.

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by:jasonrun
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oh ok thanks i didnt see that because i was focused on that proposed answer.
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linda101698 earned 100 total points
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I'm posting an answer so this information can be saved in the previously asked questions. jasonrun would like to divide the points assigned to this question among the experts he feels helped him.  I will add points to his account for him to post questions for those experts.

Linda Gardner
Customer Service @ Experts Exchange
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by:jasonrun
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thanks
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by:jasonrun
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hey dpuckett there is a question in general hardware for 100 points for you
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