Advice on a RAID 5 and 10

Discussion in 'Mac Accessories' started by Pixelmage, Mar 4, 2012.

  1. Pixelmage macrumors member

    Joined:
    Aug 22, 2008
    #1
    I have a OWC Mecury Elite Pro Qx2 and 4 x 2TB Hitachi 7200RPM / 32MB Cache Desktars. I can set the device up as either a NRAID (Span), JBOD, RAID 0, 1, 5, or 10. I am connecting it to a 2011 iMac 3.4 GHz running Mac OS 10.6.8.

    I looked at the RAID configuration options. Initially, I was going to set it up as a RAID 5 but someone said it is prone to problems especially during power outtages. Is this true? If that is the case, maybe a RAID 10???

    Would a RAID 10 show up on my desktop as a one 2TB or 8TB volume.

    I would like to store work files and family photos on the RAID but do not want to have to worry about not being able to retrieve/rebuild the information if one of the disk fails or a power outtage occurs.

    Any feeback would be greatly appreciated. Thanks!
     
  2. nuckinfutz macrumors 603

    nuckinfutz

    Joined:
    Jul 3, 2002
    Location:
    Middle Earth
    #2
    RAID 10 would show up at a 4TB pool of storage.

    RAID 10 offers superior write speeds over RAID 5 but it eats up a lot of disc space. Rebuilds are much faster on RAID 10 because you don't have to calculate and stripe parity info.

    RAID 5 is ideal if you want good read since you've got all spindles pushing data but it sucks at write because each drive is constantly striping parity.
     
  3. jackrv macrumors 6502

    Joined:
    Jul 14, 2011
    #3
    Raid 10 is a stripe of mirrors. 4 2TB drives would give you 4TB of data (a stripe of 2 2TB Mirrors - a set of Raid 1's in a Raid 0). A RAID 5 array would give you a 6TB Array. The 10 array would let one drive in each mirror fail, the 5 would let only 1 drive fail. It is the tradeoff between redundancy and storage.

    Assuming you have a good backup plan (data in 3 places), you can go either, depending how much redundancy you need without restoring. If your backups are really set in stone, you can just go with a RAID 0 and maximize the full 8TB, but this will take restoration in case of ANY failure. If raw speed isn't a factor (normal use, iTunes, etc...), either a 5 or a 10 would be fine, with Raid 5 giving you 2 TB more of space.

    EDIT: Nuckinfutz beat me to it, also adding speed info, which may be useful.
     
  4. throAU, Mar 4, 2012
    Last edited: Mar 4, 2012

    throAU macrumors 601

    throAU

    Joined:
    Feb 13, 2012
    Location:
    Perth, Western Australia
    #4
    An intelligent RAID10 controller (or software implementation) will read from all spindles just like raid 5.

    If you can afford to give up the disk space (you get 50% of the raw capacity from RAID10), RAID10 is the way to go.

    RAID 5 sucks for random small writes, because before you can do a write, you need to read from all spindles to re-calculate the parity and re-write it.

    RAID10 can do block updates in place without needing to read from all disks first and then write when updating (because there's no parity to calculate). If you do a lot of small random write IO, this can be much much faster. e.g., with 4 disks in RAID5, if you want to update one block on say, disk 3, you need to read from all 4 disks (as the parity will depend what data is on the other disks in the stripe), recalculate, and then re-write to all 4 disks. To update a block on RAID10, you simply write to two disks.


    I suspect you'll find that these days, most have more than enough capacity. Adding capacity is easy. Adding speed is not, and is a lot more expensive... :)


    with regards to RAID5 and power outages, this info may be of use:
    http://en.wikipedia.org/wiki/RAID_5_write_hole

    Essentially, if you run RAID5, you'll want to make sure either the controller in your RAID box has a battery backed up controller, or you have a UPS for it (if data integrity is important to you). The risk of hitting the RAID write hole is quite small, and if you have backups of course you can restore from backup. But, the risk is there. Then again, if your computer is hard crashing during a power outage, there's still the potential for data loss due to open files not being closed properly, etc. The raid 5 write hole is more of a concern for enterprise storage arrays...
     
  5. monsieurpaul macrumors regular

    Joined:
    Oct 8, 2009
    #5
    Be aware that RAID is not for backup. RAID gives you drive redundancy, that is if one of the drive fails, you can replace it with a minimal downtime. But if your RAID controller fails, or your file system get corrupted, you may lose all your data.
    Do you plan to have data that will be stored only on your RAID system? If so, you should add a backup.
     
  6. Pixelmage thread starter macrumors member

    Joined:
    Aug 22, 2008
    #6
    Thanks for all the great responses!

    I think a RAID 10 might be the way to go for now. It sounds like any power outage will be bad for a RAID 5 setup. And I can't afford a battery backup unit at the moment. How would a power outage affect a RAID 10?

    Also, I found a diagram for a RAID 10 setup off of OWC's Web site.

    http://eshop.macsales.com/shop/hard-drives/sata/RAID_Guide/Learn_About_RAID_10

    So how would it actually work inside the unit? Would it set up the first two bays as a RAID 0 and the next two as a RAID 1?

    How many drives can fail without losing data integrity and still be operational (reads and writes)? If it is set up based on my assumptions above. Would it be one drive from "either" RAID 0 or RAID 1 at minimum. And at most one drive from "each" RAID 0 and RAID 1 setup.

    I understand replacing a dead drives should be done ASAP but how long can a RAID 10 remain operation if a drive goes down?

    As for a backup scheme for the RAID, I am thinking a single FW 2TB external drive and just keep an eye on making sure it does not get over filled since the RAID 10 will be 4TB.
     
  7. monsieurpaul macrumors regular

    Joined:
    Oct 8, 2009
    #7
    To my knowledge, a RAID 10 is 2 RAID 1 volumes put in RAID 0. Theoretically, you can lose 2 hard drives (one in each RAID 1 cluster) without losing data, and having your system still functional.

    RAID 10 configuration are recommended for database server requiring high performance and high failure tolerance. IMHO, setting up a RAID 10 just for storing files is asking for trouble, especially if you can't afford power protection and a serious backup solution.
     
  8. Pixelmage thread starter macrumors member

    Joined:
    Aug 22, 2008
    #8
    Still a lot of food for thought for either RAID scenario. How would big business back up RAIDs I wonder? I think my last place of work, they utilized a N-RAID/JBOD setup.

    Eventually I will get a battery backed up controller. So it looks likes from what I am reading here, if I had a RAID 10, I have a maximum of two allowable drive failures but with a RAID 5, only one.

    Since a RAID 10 is a combination of RAID 0 and 1. Ideally if you have two drive failures, it should be in each setup, 0 and 1, to maintain operation, correct? But what happens if the two drives that fail are in the same RAID? Would it still be operational or is all your data completely cooked?
     
  9. throAU, Mar 5, 2012
    Last edited: Mar 5, 2012

    throAU macrumors 601

    throAU

    Joined:
    Feb 13, 2012
    Location:
    Perth, Western Australia
    #9
    If you can afford RAID, you can afford a UPS, or at least a dedicated backup.

    AS mentioned, raid is not a backup, it is a measure to continue operation in the event of hardware failure, and gain increased speed over and above what a single drive can provide. However, controller failure, theft, double drive failure, etc still means your data is gone. As does software/data corruption through power failure, etc.

    Put it this way; it doesn't matter how fast your drives are if they don't reliably store your data. If your data is important, buy a UPS, a dedicated backup location and THEN worry about making the machine faster or able to work through component failure with RAID.

    RAID10 is really no worse in a power failure than no RAID. Or rather, a single drive is "just as bad" as RAID is in a power failure. RAID 10 is a RAID0 stripe across 2 (or more) RAID1 disk groups.

    Irrespective of whether the controller writes to the array fully, all your apps will crash hard without closing files and flushing in-flight data to disk properly. I.e., the controller can only do what apps tell it to do - if the apps suddenly hard crash, the controller may not have updated the application data fully as it was never told to do so.


    edit:
    big business will typically have a "SAN" - a big box of disks with a dedicated controller that other machines connect to over a dedicated storage network (fibre channel, iscsi, etc). The only way to back these up is either via site-site replication of snapshots (to another SAN at a different location), or tape.

    A baby SAN may have say, 16 disks in a RAID 10 or RAID50 (striped RAID0 across multiple RAID5s) array. Larger SANs may have hundreds or thousands of disks.

    We're about to upgrade from our baby 16 disk SAN (14 disks in a RAID50 with 2 hot spares) to something with 48 disks, even the baby iSCSI SAN we have, has >72 hrs of battery backup on its storage controllers (emails a warning if it has less, lol), the new one we're about to procure has a mix of SAS, SATA and SSD drives in several RAID arrays to keep hot data on fast disks or SSD cache, and less frequently used data on SATA. This box is going to be RAID5 with quite a bit of SSD caching and battery backup.

    However, ideally for maximum speed (if you can afford the cost of more drives), even in the enterprise, RAID10 wins. You can create massive RAID 10s with a RAID0 striped across tens (or hundreds) of RAID1 groups....


    AS to how long a RAID10 can go with a disk failure? Well, if it is a RAID 10 of 4 disks, (2x 2 disk RAID1s), you will work in a degraded state until the other disk in the same RAID1 group fails. Then, you're screwed and need to go back to backups. So, you NEED to know when a disk fails.

    If you have say, a 6 disk RAID 10, with 3 disks in each RAID1 group you can handle up to 2 drive failures in each RAID1 before failure. 4 disk RAID1 groups = 3 failures, etc. In practice, the most common setup is 2 disk RAID1s (or if you have REALLY mission critical data, like say - a bank, 3 disk RAID1s) and some hot spares in an array.

    Bear in mind, however that if one of your disks fails, and the other disks are of the same make/model, its quite possible you'll see another disk failure quite soon. Especially if they failed due to over-temp, power spike, etc... It is best to replace "ASAP". In fact, enterprise arrays include "hot spares" to automate this process - essentially a drive fails, is identified by the RAID controller, the hot spare takes over and starts rebuilding automatically, and the SAN emails the admin (or storage vendor) to order a replacement disk...

    Hot spares are especially important with RAID5 or RAID50 - as when rebuilding or running in a degraded state, the remaining disks must work much harder to rebuild the data, and rebuilding takes a lot longer than with RAID1 or RAID10. Pushing disks hard is more likely to make one that may be slightly flaky, fail. 2 failures in a RAID5 group = go back to tape.... so you want to rebuild ASAP...
     
  10. FireWire2 macrumors 6502

    FireWire2

    Joined:
    Oct 12, 2008
    #10
    Raid5

    I'm totally disagree about RAID5 need a battery backup.

    I have one of this eBOX-R5 set as RAID5 I can turn ON/OFF at ANYTIME I want

    Here is another user using the same RAID5:
    http://vimeo.com/29501754
     
  11. Pixelmage thread starter macrumors member

    Joined:
    Aug 22, 2008
    #11
    1. It sounds like no matter which RAID I end up using, if I am not using a UPS, data corruption is possible. Would the data corruption effect all the data or just the data that was being written at the time of the power failure (and the rest of the data is fine)?

    2. Which UPS are Mac friendly?

    3. Just wondering, does a power failure effect a boot drive the same as the drives in a RAID?

    4. A RAID helps maintain workflow and quick access to data. What would be the best way to back up the data from a RAID to another single external drive? Can this be done with Apple's Time Machine or some other software like Super Duper or Carbon Copy Cloner?
     
  12. flynz4 macrumors 68040

    Joined:
    Aug 9, 2009
    Location:
    Portland, OR
    #12
    1. I would strongly recommend getting a UPS. I have 4 APC units and 1 Triplite unit. Mac's recognize all of them.

    2. See above

    3. All drives are subject to data corruption with power failures. I live in a place with almost no power failures (1 unplanned outage in 18 years)... but I still us a UPS for all computers. If you have direct attached storage, I think it is OK to keep the DAS pugged into the same UPS as the computer. I set my computers to turn off after a 5 minutes power failure, or 50% remaining on the UPS, whichever comes first. This guarantees that there is a lot of power remaining for any DAS. Since the computer is shut down, there will be no disk traffic, so a power loss to your DAS will not corrupt the drive. The only possible exception is if a rebuild or other disk maintenance is underway. Most good NAS/DAS boxes have enough reserve power (super caps or such) to empty their write buffers on a power fail.

    4. This is the $50,000 question. First, are you using your RAID box as a NAS or a DAS? Personally, I think that NAS boxes are best for backup destinations, because they are always available to your laptops when you are home. You want your backups to run automatically without human intervention. Time Machine can theoretically back up to various NAS boxes, but sometimes Apple changes things and they lose compatibility. For that reason, I use Time Capsules as NAS boxes for backup destinations.

    For my iMacs, I prefer either enough internal storage, or DAS. My preference for DAS over NAS is because it is easier to backup. I use both TM and Crashplan+. Both will back up local (internal or DAS) storage... but neither do NAS (as far as I can tell). I just got a 8TB Promise Pegasus R4 Thunderbolt drive to play around with. So far, I have been running speed tests using my TB equipped MBA, and am very impressed with its speed. As a ThunderBolt device, it is really a DAS. My personal iMac does not have TB yet, but I will be getting a new iMac once they are released. I plan to attach the R4 to that machine. I have not determined for sure if I will use it... it really depends on how large of SSD and HDD is available for the new iMacs.

    Option 2 is to pick up a cheap MacMini server, and attach the Pegasus R4 to the mini. That will give the server a lot of local DAS (6TB using RAID5)... and as a server, it can serve all of the computers in the house. Also, since it is a Mac server... I will be able to run any Mac based backup program on the server... so I can keep using TM and CP+. Ultimately, I think I will go this route, but I may wait a bit before buying yet another Mac for the house.

    Most NAS and DAS raid boxes allow you to attach external drives for backup. This is not a speed critical operation, so a cheap 4TB USB drive is probably sufficient. The real difficulty of backing up many NAS boxes is their relative inflexibility of backing up to the cloud. Some have a built in utility, but in most cases, the options are limited. Once again... this is why I prefer a DAS. You get all of the backup flexibility of the computer the DAS is connected to, so backup of the DAS is pretty trivial.

    /Jim
     
  13. FireWire2 macrumors 6502

    FireWire2

    Joined:
    Oct 12, 2008
    #13
    Not really! depend on your raid controller. If you have write-through enable then there is no affect what so ever...

    Just Wed night my work building power is shut down I have not problem seeing my RAID later in Thursday morning...

    I believe the more cache in your raid controller, the worst data lost you will have.

    My eBOX-R5 over two year in service here NEVER has UPS and never lost/corrupt data. We have maybe 3~4 times power outage in mid of the day... Nothing happen to our raid volume
     

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