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Thin Client and Virtual Machines Thread, SAN kit purchased, no turning back! in Technical; Originally Posted by AButters Have you benchmarked this yourself? In my systems RAID50 batters raid 6 for read and especially ...
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    SYNACK's Avatar
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    Quote Originally Posted by AButters View Post
    Have you benchmarked this yourself? In my systems RAID50 batters raid 6 for read and especially write speed. It's also more reliable generally.

    There is no place for Raid 5 based on it's awful scalability and poor write performance.
    It depends on the hardware, the cache and the usage profile. You are unlikely to need performance levels higher than 6 and you loose huge amounts of space. 5+0 can actually have less reliability as 6 will cope with any two drives failing, 5+0 can be killed by a two drive failure 50/50 odds. Raid5 is one of the most scalable sets but I agree with a large number of drives 6 should be the minimum. I'd trust 5 over 1 or 1+0 any day.

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    MrWu (6th July 2012)

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    I might be wrong but just stuck 12 x 600Gb Drive in the RAID calculator for RAID 6 and RAID 50 and they both spit out 6TB..is the calculator incorrect?

    What I could do though is use RAID 6 for the 600Gb drives and RAID 10 or RAID 50 for the 300gb ones. for mixture of performance and resilience. :-)

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    Quote Originally Posted by MrWu View Post
    Have the 12 HDDs divided into seperate RAIDS according to SQL / Exchange etc..but risk of over complication.
    We've split off SQL Databases and Exchange Databases into dedicated RAID10 groups, then use another RAID 10 set for all the Host OSes.

    I don't see that it complicates things etc, but it does mean that you need to have some extra disks over creating one giant 'super group'.

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    Quote Originally Posted by MrWu View Post
    I might be wrong but just stuck 12 x 600Gb Drive in the RAID calculator for RAID 6 and RAID 50 and they both spit out 6TB..is the calculator incorrect?

    What I could do though is use RAID 6 for the 600Gb drives and RAID 10 or RAID 50 for the 300gb ones. for mixture of performance and resilience. :-)
    My bad was reading it as 5+1 instead, 6 is still more redundant.

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    Quote Originally Posted by SYNACK View Post
    It depends on the hardware, the cache and the usage profile. You are unlikely to need performance levels higher than 6 and you loose huge amounts of space. 5+0 can actually have less reliability as 6 will cope with any two drives failing, 5+0 can be killed by a two drive failure 50/50 odds. Raid5 is one of the most scalable sets but I agree with a large number of drives 6 should be the minimum. I'd trust 5 over 1 or 1+0 any day.
    Hmm its not as straight cut as that, as RAID5 and RAID6's overall reliability depend on the number of, speed of and size of disks used. In both R5 and R6 cases the more slower, larger disks used in one array the greater the chance of total loss of the array. As so many people seem to favour using large numbers (10+) of large SATA drives in Raid 5 or 6, they are seriously risking a total faliure (especially with RAID5) as if one drive goes down the time taken to rebuild the array onto the replacement disk is so high that the chance of another disk failing whilst the array is being rebuilt are alot higher.

    With RAID50 you can keep expanding the number of disks in the array in groups of three and the overall chance of a total faliure stays the same as if you had just 6 disks. It's predictable.

    Raid6 is good, don't get me wrong, but the rebuild speed penalty on large arrays combined with generally poor write performance for me is absoloutly not worth the extra free space gained by using RAID6. Your milage may vary.

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    It's also worth pointing out that you can customise how much disk space you actually lose on RAID50 by increasing the number of disks in each raid 5 group.

    E.g if I had 18 x 300GB drives set up as RAID50, I could either go with 6xsets of 3disks RAID5 all in RAID0 (and lose 6 disks worth of space) or I could go with 2 x SETS of 9 disk RAID5 all in RAID0 and only lose 2 x disks worth of space. The former will offer much better write performance at the cost of more disk space, the latter will offer less perfromance but you lose less disk space. So the choice is yours, RAID50 is very customisable.

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    Quote Originally Posted by AButters View Post
    Hmm its not as straight cut as that, as RAID5 and RAID6's overall reliability depend on the number of, speed of and size of disks used. In both R5 and R6 cases the more slower, larger disks used in one array the greater the chance of total loss of the array. As so many people seem to favour using large numbers (10+) of large SATA drives in Raid 5 or 6, they are seriously risking a total faliure (especially with RAID5) as if one drive goes down the time taken to rebuild the array onto the replacement disk is so high that the chance of another disk failing whilst the array is being rebuilt are alot higher.

    With RAID50 you can keep expanding the number of disks in the array in groups of three and the overall chance of a total faliure stays the same as if you had just 6 disks. It's predictable.

    Raid6 is good, don't get me wrong, but the rebuild speed penalty on large arrays combined with generally poor write performance for me is absoloutly not worth the extra free space gained by using RAID6. Your milage may vary.
    But if you loose two drives across two stripe groups and your toast, you can loose any two under raid6. The set size is an issue with large slow drives for all types.
    Last edited by SYNACK; 6th July 2012 at 01:25 PM.



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