Why Do You Use RAID 1?

Discussion in 'Desktop & Laptop Computers Forum' started by KraGorn, Sep 25, 2004.

  1. KraGorn

    KraGorn
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    I'm not asking how RAID works, what I'm interested in is why people use it. Let me explain.

    RAID 0 is fine, you use it for performance and to make a larger file-system so to consolidate all available space. The alternative of course is JBOD which has the advantage of not requiring identical disks.

    RAID 5 is fine, you get performance and redundancy and for 'real-time' updating applications it's probably mandatory.

    However, IMHO RAID 1 is nearly pointless. Why?

    Well, it's a substitute for RAID 5 in many situations but given disk costs these days then the extra paid for a parity disk is worth it in most cases I'd suggest.

    For all systems where files are largely static then IMO RAID 1 is NOT a good idea. Sure, it guards against disk failure but what it doesn't protect against are those kinds of problems which in my experience occur far more frequently than disk failures: software crashes causing file corruption and far more likely the "whoops, didn't mean to delete that" syndrome.

    In HTPC applications we're talking about huge files but not ones that change once they're created. In this application I think using simple disks and a timed backup program, like Second Copy I use or one of many other such, provides not only protection against disk failure but also most of the 'accidents' which can occur that RAID 1 doesn't protect against.


    I put this thought up for those considering using RAID 1 as something to consider. Simply because your mobo can support RAID 1 doesn't mean it's a good idea to use it .. IMHO, of course. :D
     
  2. MikeTV

    MikeTV
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    I am not a raid user myself, mainly because I agree with your points. I can see an advantage in a datacentre environment, but in the home/office, it seems like overkill.

    However I can appreciate the performance advantage, particularly for video streaming/capture situations, and perhaps it is useful for consumers of high definition material. This really just comes down to bandwidth - if faster disks were available, it would achieve the same result.

    But maybe there are some scenarios that I have overlooked.
     
  3. GrahamMG

    GrahamMG
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    Raid 1 gives no performance advantages, it is just a mirror of the other drive so makes a hard disk failure (when, not if, it happans) far less painful...Raid 1 is built onto most motherboards these days, Raid 5 isn't probably due to space requirements inside the PC and that most motherboards only have connections for two disks....

    When your hard drive goes tits up, you have a smug feeling if it was part of a Raid1 (or 5) as you just fit another and carry on (most of the time you can use it before you replace the broken disk), if you use Raid 0 for performance you double the chance of loosing data........
    A proper backup routine is of course the correct way to do things but hardly anyone does it enough.......
    I use Raid 1 in the HTPC (and anything else that has the facility) and then nail a big NAS (raid5) on the network for central file storage (in the loft where noise isn't an issue). I don't worry about disk failure at all and with a 1Tb music collection that is a serious issue...
     
  4. KraGorn

    KraGorn
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    To be honest, video streaming either on or off disk is well within the capability of SATA devices. Where performance could be a problem is at the channel level.

    As Graham alluded to, if performance is an issue then RAID 0 is the choice as it uses a technique called 'striping' which alternates blocks of a file between each disk in the array. So, as long as each disk is on a separate controller you effectively add each channel's bandwidth together .. it's not quite the sum total but it's close enough.

    Yes, but what happens when the O/S sneezes and corrupts the file system, RAID will faithfully propogate the corruption across all copies. :cool:

    Actually, I can see the point of using RAID 1 for a system partition for resiliency reasons, but it's data storage I'm primarily suggesting RAID 1 is rarely useful.
     
  5. rhinoman

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    It depends what your trying to store, my work files (small word and excel docs) that are used frequently but dont take up much space but do require the safety of a hardrive failure, but with the abilty to not have too much downtime are the ideal candidate for raid 1. In 8 years they have only ammassed about 10gb, 2*40 gb drives from the motherboard raid is all that is needed. The cost requirement for raid 5 and multiple disks are not needed.

    That said I also use raid 5 controllers for my music/dvd/hi-def collection for the sheer time required to re-rip stuff if a drive died in a JBOD.
     
  6. KraGorn

    KraGorn
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    For the stuff that's not re-rippable ( ;) ) do rely solely on the RAID 5 array for sceurity?

    Given that re-creation from original sources is possible then I can certainly see RAID 5 .. or perhaps even RAID 1 I guesss .. is sufficient, but if something isn't re-creatable I prefer to rely on manual copying to avoid those 'whoops' situations.
     
  7. HMHB

    HMHB
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    There have been a few articles recently suggesting that RAID 0 doesn't give any increase in speed in a lot of cases.
     
  8. lmccauley

    lmccauley
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    Yes, it's probably better to perform regular backups for exactly this reason.

    RAID 1 can give you better read performance, but I don't think this would apply to reading a single, large file. More appropriate for multi-user environments (reduces the head movement caused by different users wanting to read different files).

    Someone told me that it could be risky using non-identical disks in a RAID 1 configuration. Obviously, when you initially set up your array, you'll use 2 identical disks, but what happens in a few years time if 1 goes and you can't get an identical replacement? I guess at that point you would probably buy 2 new disks (which would probably have a larger capacity), and copy the data from the original survivor onto them.

    Cheers,
    Liam
     
  9. KraGorn

    KraGorn
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    To some extent the requirement for identical disks is probably more important if you use hardware RAID as with software there's an extra layer of abstraction.

    Also, what's an 'identical' disk? For instance I just put to 120Gb Barracudas I had lying around in this machine .. with identical part numbers they're different sizes, either one had 3Mb of bad sectors mapped out in the factory or else there's a subtle difference in geometry .. they were made only a few months apart so it's kind of hard to ensure identical drives.
     
  10. Stuart Wright

    Stuart Wright
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    I've just ordered a brand new PC. It's at the core of my business and gaming universe.
    I'll be building it myself and the crucial part of the spec is using 4 hard drives to create 2 sets of RAID 1 discs.
    The first pair of discs are WD 74 gig Raptors for great speed for the OS and essential apps.
    And the second pair are WD 200 gig drives. All SATA, of course.
    The motherboard I have ordered supports 2x4 lots of drives.
    Put simply if either of my drives crashes, losing, for example, my outlook file, websites, marketing info advertiser details etc., my business would suffer because of the lost time in recreating that information.
    It's absolutely critical that if 1 disc fails, I still have the information.
    RAID 1 - essential.

    As an aside, the 925 chipset is supposed to support 'command queueing' for SATA drives, so they should be super quick.
     
  11. lmccauley

    lmccauley
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    Ah, have to admit, I tend to think of hardware RAID.
    Exactly!

    Cheers,
    Liam
     
  12. lmccauley

    lmccauley
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    I thought we were just talking about HCPC, though. I use RAID at work (data striped across 6 disks, all mirrored to another 6 disks) because it is critical for the customer to be up 24/7. We also do hot backups nightly in case the OS (unlikely - it's UNIX) or RDBMS (more likely - it's Oracle) or users (most likely) decide to corrupt/lose the data.

    As an aside, if the machine is critical to your business and if you have the space, I would recommend backing up your critical, changing files to another location on disk before copying them off to tape/CD/DVD, as well as testing that your backups are working (by rebuilding your machine using a backup) on as regular basis as you can manage. I saw a system go down and the database become corrupted, only for the system admin to find that the backups hadn't worked properly for the last few months - so all his backup tapes were useless. Would have been a huge disaster if it wasn't for the fact that there was still a redundant process running every night that copied the database files off to an unused area.

    Cheers,
    Liam
     
  13. ukbubs

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    Video files - on a 1.4tb raid 5 array. No backup (that cost plenty!)

    O/S on my 'production' machine - RAID 1 (SATA hardware RAID), plus a backup on another machine. Reason - It takes several days to load all the myriad apps, then tweak them and the desktop to be just the way I like things.

    I take the the point about O/S corruption - I guess at the end of the day you pays your moiney and .....etc etc.

    Thus the backup is a last line of defence.

    Timely thread 'tho - am just about to mvoe to SCSI, but I'll put my question in a new thread

    :)
     
  14. lmccauley

    lmccauley
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    Exactly - it's all just risk management at the end of the day. Identify risks, how serious they are, how likely they are to occur, how they can be mitigated, costs, etc.

    At one extreme you've got the typical PC user with absolutely no backup / mirroring. At the other extreme, you've got an off-site (preferably in another part of the country, so won't be affected by the same flood or earthquake) duplicate system that is frequently synchronised. We usually fit somewhere in between...

    Cheers,
    Liam
     
  15. GrahamMG

    GrahamMG
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    Well that started a bit of a debate, blimey.....

    So we all largely agree then? RAID 0 isn't a very good idea, RAID1/5 is better with separate partitions for OS and data (better still separate RAID's). backups are a thorny subject as the data involved will be "hard to replicate" (i.e. hundreds of hours of ripping) and very very large.... backing up to tapes is slow, slow and well, slow (read expensive) and maybe not so secure as one chap above mentions (I have seen that lots of times as well....). How about what I do as an idea for the more adventurious? I have three pc's at home all networked together with an additional NAS unit. So all the PC's have RAID 1 and every one has a separate partition that holds a drive image of the OS which is set to backup automatically every couple of days (could do every night if I was that paraniod) on a 5 day incremential then last in first out scale......They also have one full backup a week written to the NAS overwritten weekly (ok so I am paraniod), the NAS itself has removable drive bays so all I did was fit the same type of drive bay into one of the PC's and drive image the video/music bit of the NAS onto another 4 removable disks (drive image can span disks nicely), I then pack them up and store off site, refreshing them once a month. Now assuming the bomb doesn't go off I can get back up and running without much trouble from any distaster.....

    The bit about disk drives being the same is valid (especially with SCSI) but modern SATA raids (and the latest IDE ones) seem to be far less fussy as long as the drive is the same size or bigger (easy to do these days), i'll back the video/music up to dual layer DVD one day (make that several days) when the technology settles down (and the cost drops) but as we archive a lot of stuff here at present the longevity of the DVD backup is not fully understood or proven (CD is more stable for long term storage presently). Nothing is foolproof but food for thought maybe?
     

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