[zfs-discuss] Re: Write Caching Without BBU

Gordan Bobic gordan.bobic at gmail.com
Sun Aug 18 14:24:08 EDT 2013


On 08/18/2013 07:00 PM, Niels de Carpentier wrote:
>>
>> If the controller correctly handles flush commands (i.e. ignore them
>> when a BBU is present, but honor them otherwise), then it doesn't matter
>> if it reorders writes or not - the result will always be correct,
>> up-to-date and consistent.
>
> That's a very big if. I don't see any indication or reason that the write
> back cache behaves different with or without a BBU. They just state that
> there are no guarantees if you use WB without a BBU.
>
> The just say that if you want reliable data use a BBU or WT cache.
> The need for specific WB cache behaviour without a BBU is very rare and
> probably limited to a few hobbyists. Seems reasonable to me, BBU's are not
> that expensive if you care about your data.

Just to quantify that statement:
8-port LSI SAS HBA: ~ £140
BB cache module (a DIMM with a battery port): ~ £300

They are not inexpensive.

Regardless, I think the most important point this thread produced is 
that ZFS effectively deprecates caching controllers.

Gordan

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