ZFS...

Michelle Sullivan
2019-05-03T13:07:51Z

Michelle Sullivan
http://www.mhix.org/
Sent from my iPad

> On 03 May 2019, at 22:51, Kevin P. Neal <kpn at neutralgood.org>
wrote:
>
> On Fri, May 03, 2019 at 08:25:08PM +1000, Michelle Sullivan wrote:
>>> On 03 May 2019, at 20:09, Borja Marcos via freebsd-stable
<freebsd-stable at freebsd.org> wrote:
>>>
>>>
>>>
>>>> On 3 May 2019, at 11:55, Pete French <petefrench at
ingresso.co.uk> wrote:
>>>>
>>>>
>>>>
>>>>> On 03/05/2019 08:09, Borja Marcos via freebsd-stable wrote:
>>>>>
>>>>> The right way to use disks is to give ZFS access to the
plain CAM devices, not thorugh some so-called JBOD on a RAID
>>>>> controller which, at least for a long time, has been a
*logical* ?RAID0? volume on a single disk. That additional layer can
>>>>> completely break the semantics of transaction writes and
cache flushes.
>>>>> With some older cards it can be tricky to achieve, from
patching source drivers to enabling a sysctl tunable or even
>>>>> flashing the card to turn it into a plain HBA with no RAID
features (or minimal ones).
>>>>
>>>> Oddly enough I got bitten by something like this yesteray. I
have a machine containing an HP P400 RAID controller, which is nice enough, but
I run ZFS so I have made the drives all into RAID-0 as being as close as I can
get to accessing the raw SAS drives.
>>
>> I got bitten by that on this hardware originally...  switching to
raid-0 and separate drives then switching to write-through (not write back and
definitely not write back with bad bbu) seemed to solve it.
>
> I have an old Dell R610 with a PERC 6/i and Megaraid SAS driver Ver 4.23.
> When I use mfiutil to set caching to write through it still goes through
> the cache. Which means that if a drive fails and the machine reboots the
> firmware stops the boot because it has data in the cache that it wants
> to store on the failed drive. So a normal failure of a drive in a three
> way ZFS mirror that shouldn't cause a loss of service actually does.
>


I don?t think it will do that in write through.. it will everytime in write
back.

> Thumbs down to RAID cards.
> --
> Kevin P. Neal                                http://www.pobox.com/~kpn/
>
> "Good grief, I've just noticed I've typed in a rant. Sorry
chaps!"
>                            Keir Finlow Bates, circa 1998
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