ZFS RaidZ2 with 24 drives?

[Thomas Burgess]

On Wed, Dec 16, 2009 at 3:43 PM, Matt Simerson <matt at corp.spry.com> wrote:

>
> On Dec 15, 2009, at 3:52 PM, Solon Lutz wrote:
>
>  I deployed using the two configurations you see above. Both machines
>>> have a pair of Areca 1231ML RAID controllers with super-sized BBWC
>>> (battery backed write cache). On back01, each controller presents a 12-
>>> disk RAID-5 array and ZFS concatenates them into the zpool you see
>>> above. On back02, the RAID controller is configured in JBOD mode and
>>> disks are pooled as shown.
>>>
>>
>> Why concatenate them into one pool and give up the redundancy?
>>
>
> I didn't need redundant redundancy.
>
>
>  I have the same setup: Areca 24-port RAID6 (24x 500gb)
>>
>>       NAME        STATE     READ WRITE CKSUM
>>       temp        ONLINE       0     0    24
>>         da0       ONLINE       0     0    48
>>
>> And it very nearly killed itself after 28 months of flawless duty...
>> All went fine until 4 drives disconnected themselves from the Areca due
>> to faulty SATA-cables. This crashed the Areca in such a way, that I had
>> to disconnect the battery module from the controller in order to get it
>> initialized during boot-up.
>>
>> Cache gone - ZFS unable to mount 10TB pool - scrub failed - I/O errors
>>
>> This was three months ago and if I hadn't found an extremly skilled person
>> who was able to manually find and distinguish between good and corrupted
>> meta-data sets, replicate them in their proper spots and zero out corrupt
>> transaction ids - I would have lost 10TB of data. (No backups - to
>> expensive)
>>
>> Why do you use JBOD? You can configure a passthrough for all drives,
>> explicitly degrading the Areca to a dumb sata controller...
>>
>
> Why would I bother?  Both ways present each disk to FreeBSD.  Based on my
> understanding (and an answer received from Areca support), the only reason
> I'd bother manually configuring some disks for passthrough is if I wanted to
> use some disks in a RAID array and others as raw disks. Configuring JBOD
> mode configures ALL the disks on the controller as passthrough devices.
>
>
I think the main reason is that ZFS is better when it has raw drives.  Some
of the features of ZFS don't work as well without having access to the
drives in this way, and other features don't work at all.

In general, it's always best to let ZFS handle the raid stuff and not use
the hardware raid settings.







> Matt
>
>
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