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      #1  
    Old 01-23-01, 07:05 PM
    Dave's Avatar
    Dave Dave is offline
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    Post your comments on our article entitled: "RAID: An In-Depth Guide To RAID Technology"

    Link: http://www.systemlogic.net/articles/01/1/raid

    Remember, you do not need to register to post comments, but it is recommended you do.

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      #2  
    Old 01-24-01, 12:23 PM
    MR2
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    Good article on the topic, more in depth than the more usual "basics". A persistant question that I have not been able to get answered: How valuable is the hard drive cache when used in the various forms of RAID (HW vs. SW) & (0, 0+1, 1+0, 5)? What about the RAID controller cache? Is there a good "rule of thumb" for determining how much RAID controller cache is needed? Some other questions: When would you choose (performance wise) between an on board RAID controller vs. a PCI (33 or 66)?
    Thanks.
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      #3  
    Old 01-24-01, 12:34 PM
    KT
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    System builders and administrators that plan on implementing a RAID solution need to analyze the applications utilizing the disk subsystem. RAID 5 suffers from a "small write problem" (lots of information available on the web). Basically, an application that will send lots of small writes to the disk (accounting applications or other online transaction processing apps) will suffer due to the overhead of writes.
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      #4  
    Old 01-24-01, 01:36 PM
    Althornin
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    in your table you have the XOR function done incorrectly, and you state "Basically, if the input is the same, the output is true. Otherwise, the output is false"
    which is about as backwards as can be.
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      #5  
    Old 01-24-01, 01:43 PM
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    If the XOR Functions are incorrect, the mistake may have happened when I was converting the article into HTML. I'll get Tom (the author) to look at it and let me know what the correct table should look like.

    Thanks for noticing, if it needs to be fixed, it will be done ASAP.

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      #6  
    Old 01-24-01, 02:19 PM
    Number9
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    Hi, Good article.
    How does RAID work with SCSI?
    The SCSI drive doesnt hook up to the RAID controller, right? Im assuming since SCSI drives have different pin connections.
    Does one have to use a software RAID solution with SCSI?
    Does the SCSI controller your using determine if you could use RAID?

    thanks
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      #7  
    Old 01-24-01, 03:00 PM
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    Red face

    About the XOR function, I think I got a little confused about the 1s and 0s... originally when I looked at the truth table it was in binary, so I converted it over to T and F... mixed up my Ts and Fs... so it should be that XOR is true if and only if one of its operands is true except when both are true, then it's false... I have class now but I'll get that changed right away. Sorry for the mistake and thanks for letting us know.

    Tom

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      #8  
    Old 01-24-01, 03:29 PM
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    Tom sent me the fix, and I've made the changes

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      #9  
    Old 01-24-01, 04:03 PM
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    Talking

    About cache in a RAID controller. It pretty much acts just like the cache in the hard drive, storing recently used info, since the RAID controller makes it seem like you're using one large hard drive. More cache means better performance, but like regular hard drives, increasing the cache might not impact the performance that dramatically.

    However, if write back caching is enable, the writing performance is improved. The system doesn't have to wait for the write to complete. The controller writes it to cache and as far as the system knows, the write is done. This is good for RAID 5 or similar levels that have a high over head on writes. But if the power fails or something happens before the actual write takes place, the data can be inconsistent...

    Well, the onboard RAID controller is still on the same bus as if it were a PCI card I think. So in terms of performance, if the RAID controllers are the same, there probably is a very small difference. If the card has more cache, or better features then yea otherwise I can't see it being much different from an onboard controller... Not much to warrant choosing based on performance though... the onboard controllers are cheaper and don't take up a PCI slot so I'd probably go for that...

    Also, to answer the SCSI question... SCSI is just the interface used. SCSI drives hook up to the RAID controller. The controller basically acts like a regular SCSI controller but with the features of RAID. It's the same with IDE/ATA. Obviously you can't intermix the two. SCSI RAID with SCSI hard drives, IDE RAID with IDE drives...

    Tom


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    [This message has been edited by Tom (edited 01-24-2001).]
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      #10  
    Old 01-24-01, 04:16 PM
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    Thak you Tom; What I've been able to gleen from others regarding HDD cache vs. controller cache is the HDD cache actually hinders performance in a RAID situation to a small degree and large cache HDD's are a waste of $$$ in this situation. No one has been able to come up with a "rule of thumb" regarding how much controller cache is needed. "They" say more users = more cache, but no one can give me a starting point. One thing that I learned is splitting the HDD's up amongst different controllers. For example if you had a double, triple, or quad RAID controller; that you would put the striped drives on one and the mirrored drives on another. But you sorta covered this.
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      #11  
    Old 01-24-01, 04:25 PM
    jimntexas
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    Thanks for the excellent article on RAID. I understand the prinipals now, but you can anyone explain to me about how things work in practice w.r.t. mirroring?

    I have a small business that basically runs off a hard drive. Every transation, email, and software tool I use are on this one drive.

    Let's say I move this data to a system with two RAID mirrored drives. Now each write goes to both drives, each drive is a mirror of the other. I'm with you so far.

    Now one of the drives malfunctions, gets bad blocks, or some other intermidant fault. How do I know that any thing has happened? How do I know which drive is bad? Do I just replace the bad drive with a new blank drive and expect the RAID system to make a mirror of the good drive on the new one?

    I guess I'd like to hear how mirrored are used in practice. I know I really need to use them in my business.

    thanks

    Jim
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      #12  
    Old 01-24-01, 05:54 PM
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    Question

    So if I understands this right..
    What I need is the possibility to have two harddrives work as one partition. ex. I`ve bought a new 25gb hd, and I want to use it together with my old 13,5 gb hd. But what I want is to use them as one HD. e.g. what would have been c: (13,5gb) and d: (25gb) becomes c: (25gb+13,5gb=38,5)

    I have bought a ATA100 kontroller card, I have a 80gb drive hooked to it, do I have to have a identical 80gb hd to get the space, or does it work with an 80 and one 40gb?

    plz help, thanks!

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      #13  
    Old 01-25-01, 12:37 AM
    Ed Gillston
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    Thank you for the article. I am also a speed freak when it comes to getting the most out of my equipment. I was wondering if you had done any benchmarks between NT duplexed drives, or RAID0+1, or RAID1+0 using NT. In addition, what about the stripe size on the RAID in conjunction with NT's format of the cluster size. (512 bytes vs 64K).

    Thanks for the info.
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      #14  
    Old 01-25-01, 01:12 AM
    4665
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    Great article on RAID, but I didn't see you mention anything about Fibre Channel, which uses fibre optics for even faster performance then SCSI.
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      #15  
    Old 01-25-01, 04:15 PM
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    Talking

    To JimnTexas:
    Well it really depends on the RAID controller you're using. Usually the card comes with drive array monitoring software that will let you know the status of the drives at all times. If one of the mirrored drives fail, you will be notified. There are controllers that allow you to replace the damaged drive without having to shutdown the system. Otherwise, you have to replace the drive and rebuild the data using the provided monitoring program. The amount of downtime to rebuild a lost mirror isn't that bad. As with everything else, if you want little or no downtime, you'll have to pay more for that...


    To Dingy:
    Well if you just want to combine more than one hard drive into one, you're probably better off using software RAID. You won't be using any RAID levels, but rather what some people refer to as JBOD (just a bunch of disks) or spanning, to just combine the hard drives.

    If you want the performance increase of RAID 0 or a similar level, you'll have to use identical hard drives. It's not required but if you don't you'll just be wasting space. For example, if you use a 25gb and 13.5gb hard drive in RAID 0, the total space will be 27gb (13.5 x 2). That's because it needs two identical storage spaces to stripe correctly. The same is true for mirroring or RAID 1. The total capacity is always going to be the smallest capacity hard drive you have times the number of drives, if you're using RAID 0. That's only if you want to use RAID 0, but if you just want everything as one large hard drive, I'd go with a software RAID solution that does that.

    To Ed Gillston:
    Well duplexed drives don't really offer much of a performance increse as opposed to RAID 0+1 and RAID 1+0 since it doesn't involve striping. I haven't really seen any benches comparing RAID 10 and 01 though. I'd think the performance would be pretty close. As for stripe size, I've heard some people say that the stripe size should match the cluster size. The theory is that each cluster could fit in one stripe. However, that's never really the case. I'm not really sure how much of a performance increase you'll get. I mean there are a suggestions on how to choose stripe size. I think ir really depends on the applications, transaction types, and file sizes involved etc.

    To 4665:
    The reason I didn't mention Fibre Channel(FC-AL) is because most people and businesses aren't going to be using it. For those who aren't familiar with what it is, it's basically another interface standard like SCSI but uses fiber optic channels. I mean SCSI and Fibre Channel are not JUST interfaces and there's a lot more to it than that, but I'd rather not go into it here. Fibre Channel allows for a sort of network of storage devices. Devices can be separated like 10 kilometers apart, which is a lot more than the 30 meters that SCSI allows. The transfer rate is also in the Gbits/s. You can imagine how expensive this is... It's not as widely implemented as SCSI but I guess it's good to know that it exists.




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