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Thread: Can single drive from a RAID 1 mirror be accessed as a normal drive?

  1. #11
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    I've been using, testing mdadm for over two years with good results. I use raid5 in a four-drive array.

    My system is a dual-core Intel E8400 that's very fast, with 4GB of RAM. The CPU usage during write-heavy array access is moderate, like 15% total.

    Not knowing what your intended useage will be it is hard to say how the CPU performance might be.

    Using raid5 increasing data throughput over a single drive, like one drive's performance times the number of drives minus one. For example, if one drive has a 70MB/sec throughput, than a three-drive raid5 will have 140MB/sec, if your controller bus is other than PCI. Both PCI-X and -E will give you over 350MB/sec if your array is large enough.

    Let us know what you decide... many folks here have lots of mdadm experience, a very good program but one that has a steep learning curve, if you know what I mean. <smile>
    Regards, frank, at http://yantrayoga.typepad.com/noname/
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  2. #12
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Thanks Roger, that's very informative!

    I'm going to order a new pair of SATA drives reasonably soon so I think I'm going to initially setup a software RAID 1 server with them and experiment with that. I was tempted to build a low power server, maybe based on the Intel Atom so it's nice to know CPU load isn't really an issue.

  3. #13
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Quote Originally Posted by fjgaude View Post
    Not knowing what your intended useage will be it is hard to say how the CPU performance might be.
    It's my intention that the server will run Ubuntu Server (I run Ubuntu on all my desktop/laptop PCs so it's what I'm most familiar with) to provide SAMBA, NFS and Subversion services to myself and maybe one other person as part of a small web development business. It's to replace my ageing ~1.2GHz, 256MB RAM CentOS server which actually served me very well, but is now moving on to digital retirement. I was also hoping to eventually use it as an office IMAP mail server too, again serving only 2 or 3 people.

    Quote Originally Posted by fjgaude View Post
    Using raid5 increasing data throughput over a single drive, like one drive's performance times the number of drives minus one. For example, if one drive has a 70MB/sec throughput, than a three-drive raid5 will have 140MB/sec, if your controller bus is other than PCI. Both PCI-X and -E will give you over 350MB/sec if your array is large enough.
    RAID 5 does sound very attractive from the speed point of view, definitely something I'm going to consider, but would RAID 1 not also give a similar speed boost? Have to admit, I'm still leaning towards RAID 1 because it sounds simpler to setup! But then, I don't mind getting my hands dirty if RAID 5 will be sufficiently beneficial...

    Quote Originally Posted by fjgaude View Post
    Let us know what you decide... many folks here have lots of mdadm experience, a very good program but one that has a steep learning curve, if you know what I mean. <smile>
    Thanks, I'll certainly do that. I only heard about mdadm for the first time in the last couple of weeks so I've got plenty of learning to do!

    Many thanks!

  4. #14
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Raid1 is mirroring, with no speed increase because no stripping.

    Let us know what the CPU is on the new server.
    Regards, frank, at http://yantrayoga.typepad.com/noname/
    Homebuilt Lian-Li PC-Q33WB, Intel i7-4790K 4.6GHz, SSDs,16G RAM | Dell Laptop 11.6" N3050.
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  5. #15
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Quote Originally Posted by fjgaude View Post
    Raid1 is mirroring, with no speed increase because no stripping.

    Let us know what the CPU is on the new server.
    I had presumed because it was mirrored, data requests could be spread between the two drives, effectively doubling access times, but I guess that's what striping is for... Hmmm, makes me sway back over to RAID 5 now!

    I have been thinking about Atom processors because of their lower power usage. I'd like to build a low power and quiet server if possible, as it will be idle for long periods of time and nothing too intensive going on even at peak times. But if I'm talking RAID 5, with three drives then I don't know how 'low power' I'm going to get it and not sure if the Atom will be up the job.

    Many months ago, when I first planned to replace my file server, it was going to be a non-raid VIA processor based Mini-ITX affair, but I'll forget about that for now because I really can't see that handling everything I want it to any more. If anyone has suggestions for something with a bit more horsepower but still quiet and lower power, I'd be very grateful.

  6. #16
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Just to let you know, you cannot boot off of a software raid5 array.

    A three-drive raid5 with three big, like terabyte, drives likely will work fine with a CPU clock of about 1.2 GHz. Now to get the double one-drive throughput, your drive controller will have to be better than PCI, which would give you about 110 MB/sec for three 70 MB/sec drives in raid5.

    Things to study about software raid:

    http://shsc.info/LinuxSoftwareRAID
    /usr/share/doc/mdadm/FAQ.gz and md.txt.gz
    http://blog.tcg.com/tcg/2006/04/recovering_raid.html

    Good luck!
    Regards, frank, at http://yantrayoga.typepad.com/noname/
    Homebuilt Lian-Li PC-Q33WB, Intel i7-4790K 4.6GHz, SSDs,16G RAM | Dell Laptop 11.6" N3050.
    Oracle VBox w/ WinXP/Win8 running Xara Designer, PaintShopPro, and InDesign CS.

  7. #17
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Quote Originally Posted by fjgaude View Post
    Just to let you know, you cannot boot off of a software raid5 array.

    A three-drive raid5 with three big, like terabyte, drives likely will work fine with a CPU clock of about 1.2 GHz. Now to get the double one-drive throughput, your drive controller will have to be better than PCI, which would give you about 110 MB/sec for three 70 MB/sec drives in raid5.

    Things to study about software raid:

    http://shsc.info/LinuxSoftwareRAID
    /usr/share/doc/mdadm/FAQ.gz and md.txt.gz
    http://blog.tcg.com/tcg/2006/04/recovering_raid.html

    Good luck!
    Thanks for the info fjgaude, and thanks for the links too! They look very useful and I'll read through them this evening.

  8. #18
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Quote Originally Posted by crystaline View Post
    I had presumed because it was mirrored, data requests could be spread between the two drives, effectively doubling access times, but I guess that's what striping is for... Hmmm, makes me sway back over to RAID 5 now!

    I have been thinking about Atom processors because of their lower power usage. I'd like to build a low power and quiet server if possible, as it will be idle for long periods of time and nothing too intensive going on even at peak times. But if I'm talking RAID 5, with three drives then I don't know how 'low power' I'm going to get it and not sure if the Atom will be up the job.

    Many months ago, when I first planned to replace my file server, it was going to be a non-raid VIA processor based Mini-ITX affair, but I'll forget about that for now because I really can't see that handling everything I want it to any more. If anyone has suggestions for something with a bit more horsepower but still quiet and lower power, I'd be very grateful.
    There's two schools of thought when talking about RAID. You have your hardware RAID proponents (me) and your software RAID proponents. I have to say I have come a long way to accepting software RAID, and I have to say that each has it's own advantages.

    With hardware RAID, in my production boxes, I eliminate the controller failure argument by keeping a spare RAID controller on hand. Problem solved. I have to say that I can't remember the last time a RAID controller failed in the thousands of servers we have where I work. It certainly can and does happen. One of the advantages of hardware RAID is that it's simple. Simple to set up and simple to maintain. That's the tradeoff for having to use a separate controller card. With hardware RAID, you install your drives, boot up and enter the controller config much like you enter the BIOS of your motherboard. From there, you select what kind of RAID you want i.e. RAID5, etc., and then add the drives to the array. Done. Battery backup on high end cards allows faster journalled rebuilds and battery-backed write-back cache may improve write throughput also

    Software RAID is a little more involved and this is why I'm gonna say for you I think hardware RAID would be better for two reasons. You said yourself that you don't want to be fudging around rebuilding stuff if it breaks. Some here will shoot me for saying this but with software RAID it will take you longer to fix a problem than hardware RAID. For proof, just go through the last 5 pages of this forum (about 1 week of posts) and see how many "My software RAID is hosed" threads you can find. I doubt you will find any hardware RAID threads. Also, if you are going to use a low power CPU, software RAID takes CPU cycles because the work is being done on your CPU and not on the RAID card.

    As for RAID5 vs RAID1? Hands down RAID5. The advantage is you have peformance and fault tolerance all in one. You can lose one drive and just keep going. With RAID1 you lose half the space with no performance increase.

    For low power and quiet, I recommend AMD's Cool 'n Quiet technology. Yes, I am biased because I am an AMD partner but I have to tell you the boxes are really quiet with the modern fans. You can hardly hear them run. They're cheaper too. As for the hard drives, really, all the people here seem to think these things take a huge amount of power for some reason. In reality it's miniscule.

    Some articles of interest:

    http://searchstorage.techtarget.com/...153747,00.html

    http://linux-ata.org/faq-sata-raid.html

    http://www.linuxplanet.com/linuxplan...orials/4349/1/

    -Tim
    www.pcchopshop.net

    Hard to find and obsolete PC and server parts. "If we can't find it, it probably doesn't exist"

  9. #19
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Tim, that's just the kind of advice I was looking for! Thank you for explaining it to me so well.

    As you suggested, I have scanned through previous posts along with some general googling and software RAID does give me the impression that it's certainly going to be: 1) more difficult to setup and with a steeper learning curve, and 2) in case of problems, more time consuming and difficult to remedy.

    So, I've now got it clear in my head that I'm going to build a 3 drive RAID 5 array, connected via a decent hardware RAID controller (and with an identical controller as spare). I don't know yet if I'll be able to get all of this into a slighter bigger than average mini-ITX (medium-ITX???) case which would be nice, but with the RAID logic handled by the controller card I can still go for a lower power Intel Atom or AMD Cool 'n Quiet processor and not have to worry about lots of CPU cycles dealing with every read/write request. Sounds great to me so far!

    If anyone could recommend some decent controller cards (that don't cost the Earth) that would be smashing. And if anyone out there is doing something similar in a relatively compact, quiet and low power case/motherboard/controller/cpu setup I'd also love to hear how you've got on and with what hardware you'd recommend!

    One other thing I remember reading; I've heard it suggested that you should use drives from different suppliers or even different manufacturers to minimise the possibility you get two or more drives from the same batch, just in case you're unlucky enough to get a bad batch. Is this going over the top or is it recommended practice?

    Again, thanks for all your help, I've found this an incredibly useful thread.

  10. #20
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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    I just noticed a test of the latest Adaptec raid controllers, the quickest, the ASR-3405 costs $2800.00 and has about the same throughput as my software raid5 array, both 4-drives, about 200MB/sec.

    http://www.xbitlabs.com/articles/sto...ntrollers.html

    All the lower priced controllers in the $300 to $600 range cannot compete.

    So the future seems to point to PCI-E SATA running from motherboard controllers. That's true right now for workstations.

    Linux software raid is moving to set the path to high performance, open source, at low prices.
    Last edited by fjgaude; September 21st, 2008 at 05:15 PM. Reason: typos
    Regards, frank, at http://yantrayoga.typepad.com/noname/
    Homebuilt Lian-Li PC-Q33WB, Intel i7-4790K 4.6GHz, SSDs,16G RAM | Dell Laptop 11.6" N3050.
    Oracle VBox w/ WinXP/Win8 running Xara Designer, PaintShopPro, and InDesign CS.

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