[ML-General] hardware RAID

[Kirk D Mccann]

On Fri, Jun 12, 2015 at 9:37 AM, WebDawg <webdawg at gmail.com> wrote:

>
>
> On Fri, Jun 12, 2015 at 5:53 AM, Kirk D Mccann <kirk.mccann at gmail.com>
> wrote:
>
>> So I've noticed two things that no one has mentioned yet.  3 drives
>> running in raid 0 and rebuild times of large drives.
>>
>> Raid 0:
>> You realize that if any one of the drives that are running in raid 0 fail
>> then you lose all your data right?  The only time you want to use raid 0 is
>> when you dont care about the data and are looking for speed.  I use raid 0
>> for our build server build drive because we always have source code that
>> can be used to rebuild the builds.
>>
>> Large Drives:
>> So if you have more two drives mirroring isnt really what you want
>> because true mirroring only works with two sets.  (That could be two drives
>> or two sets of raided disks).
>> Since you have more than two drives you are going to want raid 5,6, or
>> 7.  The raid that you choose should be based on the size of your drives and
>> the class of the drives.
>>
>> If the drives you have are more than 1TB in size and they are consumer
>> grade drives then you shouldn't be using raid 5.
>> This is a problem because the likelihood of a read failure while
>> rebuilding a disk is higher the larger the drives are.  So then you have to
>> be able to handle a read failure which requires a higher raid.  Check out
>> the calculator:
>> http://www.servethehome.com/raid-calculator/raid-reliability-calculator-simple-mttdl-model/
>>
>> Also if you are using raid dont use Western Digital Green drives unless
>> you plan to reflash the firmware to make them function like red drives.
>>
>
> Do you know if the WD greens can still be reflashed?  I have read "The WD
> Green drives did allow you to disable TLER up to a point, then WD caught on
> that people were using these drives for RAID instead of their more
> expensive enterprise level drives and put and end to it. Now the popular
> choice seems to be Hitachis, as they work in RAID arrays without any TLER
> adjustments required."
>
>
> Do you know if this is true or is it just some forum BS?
>
> Check out this forum post:
https://forums.freenas.org/index.php?threads/hacking-wd-greens-and-reds-with-wdidle3-exe.18171/

>
>
>>
>> My recommendation:
>> All that being said I am a big fan of Freenas, because it uses ZFS.
>> Btrfs is great but its not quite where ZFS is yet(or at least that was the
>> case when I last looked at it).  If you go the ZFS route you REALLY REALLY
>> need to read up on how ZFS, uDevs, and vDevs work.  Because what many
>> people dont realize is you cant just add a single disk to the array when
>> you do it makes the entire array fail if that single added disk fails.
>> When you add drives you have to add drives in sets.
>>
>> I talked to one of the btrfs devs onces.  That dev was extremely against
> me talking about btrfs like it was ZFS.  That being said I still do not
> know alot about it.  Being the dev they were I am sure we are talking about
> a lot of the details that really do make a difference in the end.  I know I
> need to do more research but at the time the dev was more into making btrfs
> stable for enterprise then features.
>
>
>
>> Personally I'm paranoid about my data, I use ZFS raid z3 on two vDevs,
>> each vDev has 5 2+TB drives.
>>
> This is one of the reasons I use ZFS w/ ECC memory.  It can detect bit
> flips!
>
>
>>
>> Oh and with ZFS you can use different size drives but you waist a good
>> amount of space when you do that.
>> And lastly be sure to schedule scrubs of your drives and do it in a way
>> that the scrub will not occur while a long smart test is running.  That can
>> cause problems.
>>
>> Can I ask what type of problems?
>
According to the forums.  "It isnt able to handle a scrub, offline test and
normal traffic well."
https://forums.freenas.org/index.php?threads/scrub-and-smart-testing-schedules.20108/

Sounds like performance.  The guy who made the post (cyberjock) is one of
their forum admins and he has helped me with some difficult questions so I
trust his opinion.


>
>> -Kirk
>>
>>
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