RAID! - Storage

This is a discussion on RAID! - Storage ; I'm trying to find info on how Raid 5 works *in the case of 3 data drives + 1 parity*. I understand how exor'ing 2 data would yield a parity bit, but the 3data/1parity algorithm eludes me. I can't think ...

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Thread: RAID!

  1. RAID!

    I'm trying to find info on how Raid 5 works *in the case of 3 data
    drives + 1 parity*. I understand how exor'ing 2 data would yield a
    parity bit, but the 3data/1parity algorithm eludes me. I can't think
    of how you'd have enough info to recover, even if there were such
    a thing as a 3-input Exor.

    Is this simple? Or is there a website or paper covering it?


  2. Re: RAID!

    "_|_|_" <_|_|_@nomail.org> writes:
    > I'm trying to find info on how Raid 5 works *in the case of 3 data
    > drives + 1 parity*. I understand how exor'ing 2 data would yield a
    > parity bit, but the 3data/1parity algorithm eludes me. I can't think
    > of how you'd have enough info to recover, even if there were such
    > a thing as a 3-input Exor.


    Yes, there's such a thing as a 3-input xor. xor(x,y,z) = (x xor y) xor z.

    If you have

    parity = xor(x,y,z)

    and you lose the x drive, you reconstruct it the obvious way:

    x = xor(parity, y, z).

    Similarly for the y and z drives. Is that all you wanted to know?

  3. Re: RAID!

    On 02 Jun 2005 17:55:20 -0700, Paul Rubin
    wrote:

    >"_|_|_" <_|_|_@nomail.org> writes:
    >> I'm trying to find info on how Raid 5 works *in the case of 3 data
    >> drives + 1 parity*. I understand how exor'ing 2 data would yield a
    >> parity bit, but the 3data/1parity algorithm eludes me. I can't think
    >> of how you'd have enough info to recover, even if there were such
    >> a thing as a 3-input Exor.

    >
    >Yes, there's such a thing as a 3-input xor. xor(x,y,z) = (x xor y) xor z.
    >
    >...


    Initially, that felt counterintuitive, but you're right, Paul--it's
    just an even/odd thing, and you could add as many drives as desired.

    > Is that all you wanted to know?


    No, but while I was typing I realized that the other questions may be
    somewhat arbitrary. In the past I've always run Raid 0 with an
    'offboard' mirror. IOW, an image on an external server that's backed
    up in non-realtime. Safer against viruses/trojans, but drive failures
    have bitten a couple times.

    I figure Raid 5 should have the same read performance with a good
    controller (3ware, probably) but I'm trying to figure out how much
    impact Raid 5 will have on write performance. I realize that this is
    dependent on the type of operation, etc. and that I'd probably get
    numerous answers stating that.

    OTOH, if average writes stay a bit faster than writes to a normal
    non-Raid drive, then it could still be worthwhile. I was estimating
    Raid5's impact on writes as 25 to 50% (giving wide leeway) so it
    should still be faster. Open to comments.

    LL

  4. Re: RAID!

    > I figure Raid 5 should have the same read performance with a good
    > controller (3ware, probably) but I'm trying to figure out how much
    > impact Raid 5 will have on write performance. I realize that this is
    > dependent on the type of operation, etc. and that I'd probably get
    > numerous answers stating that.
    >
    > OTOH, if average writes stay a bit faster than writes to a normal
    > non-Raid drive, then it could still be worthwhile. I was estimating
    > Raid5's impact on writes as 25 to 50% (giving wide leeway) so it
    > should still be faster. Open to comments.


    Which write performance? Sequential (MBps) or random (IOps)?
    How many disks?



  5. Re: RAID!

    On Fri, 3 Jun 2005 12:11:39 -0400, "Peter"
    wrote:

    >> I figure Raid 5 should have the same read performance with a good
    >> controller [as Raid 0]
    >> ...
    >> but I'm trying to figure out how much
    >> impact Raid 5 will have on write performance. I realize that this is
    >> dependent on the type of operation, etc. and that I'd probably get
    >> numerous answers stating that.
    >>
    >> OTOH, if average writes stay a bit faster than writes to a normal
    >> non-Raid drive, then it could still be worthwhile. I was estimating
    >> Raid5's impact on writes as 25 to 50% (giving wide leeway) so it
    >> should still be faster. Open to comments.

    >
    >Which write performance? Sequential (MBps) or random (IOps)?


    I would guess primarily sequential. Video and audio apps. The audio
    would have to stream multiple tracks, so there would be seeks when
    filling up audio hardware's output buffers. I don't know the size of
    those buffers.

    >How many disks?


    Probably 3 + parity. Controllers are 4 port.


  6. Re: RAID!

    > I would guess primarily sequential. Video and audio apps. The audio
    > would have to stream multiple tracks, so there would be seeks when
    > filling up audio hardware's output buffers. I don't know the size of
    > those buffers.
    >
    > >How many disks?

    >
    > Probably 3 + parity. Controllers are 4 port.


    You may get a decent sequential write speed with GOOD RAID 5
    controller.

    May see some reviews to get an idea:
    http://www.gamepc.com/labs/view_cont...500s4lp&page=7
    http://www.tweakers.net/reviews/557/23



  7. Re: RAID!

    On Fri, 3 Jun 2005 15:04:38 -0400, "Peter"
    wrote:

    >> I would guess primarily sequential. Video and audio apps. The audio
    >> would have to stream multiple tracks, so there would be seeks when
    >> filling up audio hardware's output buffers. I don't know the size of
    >> those buffers.
    >>
    >> >How many disks?

    >>
    >> Probably 3 + parity. Controllers are 4 port.

    >
    >You may get a decent sequential write speed with GOOD RAID 5
    >controller.
    >
    >May see some reviews to get an idea:
    >http://www.gamepc.com/labs/view_cont...500s4lp&page=7
    >http://www.tweakers.net/reviews/557/23


    Thanks for the links, Peter.

    I had seen the first review (gamepc) some time ago. It seemed odd
    that they got such weird results on the 3ware 8000 vs 9000. I kept
    thinking that the test was flawed, or that data was scewed. If I
    recall, they also said that on-mobo RAID would be faster since it
    didn't have to go thru the PCI bus. That also seems wrong, given that
    onboard RAID is almost invariably software-based. (I don't remember
    ever seeing a motherboard with an actual RAID controller on it).
    Also, I don't think the PCI bus would be the bottleneck in most cases.

    I'll have to research the raid controllers mentioned in the second
    article. I'm only vaguely familiar with some of those manufacturers.
    I'm surprised to see that most placed higher than 3ware, which I
    thought was at or near the top. Are the other controllers really that
    much better?

    LL



  8. Re: RAID!

    > onboard RAID is almost invariably software-based. (I don't remember
    > ever seeing a motherboard with an actual RAID controller on it).


    Is HighPoint 370 (or newer) software-based?

    --
    Maxim Shatskih, Windows DDK MVP
    StorageCraft Corporation
    maxim@storagecraft.com
    http://www.storagecraft.com



  9. Re: RAID!

    On Tue, 7 Jun 2005 10:30:42 +0400, "Maxim S. Shatskih"
    wrote:

    >> onboard RAID is almost invariably software-based. (I don't remember
    >> ever seeing a motherboard with an actual RAID controller on it).

    >
    >Is HighPoint 370 (or newer) software-based?


    Are you saying there is a motherboard using the Highpoint?


  10. Re: RAID!

    > >Is HighPoint 370 (or newer) software-based?
    >
    > Are you saying there is a motherboard using the Highpoint?


    Epox D3VA. An old VIA-Apollo-133 dual-CPU mobo (like dual PIII-800).

    --
    Maxim Shatskih, Windows DDK MVP
    StorageCraft Corporation
    maxim@storagecraft.com
    http://www.storagecraft.com



  11. Re: RAID!

    On Wed, 8 Jun 2005 15:37:01 +0400, "Maxim S. Shatskih"
    wrote:

    >> >Is HighPoint 370 (or newer) software-based?

    >>
    >> Are you saying there is a motherboard using the Highpoint?

    >
    >Epox D3VA. An old VIA-Apollo-133 dual-CPU mobo (like dual PIII-800).


    I should get one of those! Actually, a benchmark on TomsHardware
    OC'd a Pentium-M (relative of PIII) and it beat the fastest P4s and
    AMDs. Intel should take note.

    If the chipset on your PIII motherboard was made by Hightpoint, there
    may well have been some processing on board. I can't imagine a modern
    'consumer' motherboard containing the equivalent of a $400 Raid
    processor, though.

    Nor can I imagine anyone doing anything but running Raid 1 on a normal
    motherboard Raid controller. I suppose there may be some advantage in
    Raid 0 if it's necessary to create a 500+gb volume, but it can't be
    very fast.


  12. Utility for determining which drive is hooked to which controller

    After having trouble tracing cables back to a given drive and
    determining which drive letter that represents, it occured to me that
    someone must have written a utility to do this already.

    IOW, show that a Seagate 400gb is attached as master on cable 1 of the
    Adaptec ATA controller, etc. Kinda like CPU-Z but for drives.

    Anyone know if this exists?


  13. Re: Utility for determining which drive is hooked to which controller

    Windows? Start/Run/devmgmnt.msc/Show Devices By Connection/expand the tree.

    UNIXen? Try "atacontrol".

    --
    Maxim Shatskih, Windows DDK MVP
    StorageCraft Corporation
    maxim@storagecraft.com
    http://www.storagecraft.com

    "_LL" <_LL@nomail.net> wrote in message
    news:092ia1d6t82ibbbq6dskibeh0tvtb90b5s@4ax.com...
    > After having trouble tracing cables back to a given drive and
    > determining which drive letter that represents, it occured to me that
    > someone must have written a utility to do this already.
    >
    > IOW, show that a Seagate 400gb is attached as master on cable 1 of the
    > Adaptec ATA controller, etc. Kinda like CPU-Z but for drives.
    >
    > Anyone know if this exists?
    >




  14. Re: Utility for determining which drive is hooked to which controller

    On Fri, 10 Jun 2005 09:39:24 +0400, "Maxim S. Shatskih"
    wrote:

    > Windows? Start/Run/devmgmnt.msc/Show Devices By Connection/expand the tree.
    >
    > UNIXen? Try "atacontrol".


    Brilliant. Thanks, Maxim. Embarrassing that I never noticed that
    option in devmgmnt before.

    In the course of looking around, I found this program:
    http://www.passmark.com/products/diskcheckup.htm
    It doesn't do specifically what I wanted, but it may be of use
    to some. It does show detailed info on drives. Unfortunately,
    it doesn't find devices on other controllers (3ware and Adaptec
    PCI controllers, both mapped as SCSI).



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