geom_raid5 inclusion in HEAD?

韓家標 Bill Hacker askbill at conducive.net
Wed Nov 7 05:09:02 PST 2007


Ivan Voras wrote:
> On 07/11/2007, Arne Wörner <arne_woerner at yahoo.com> wrote:
> 
>> graid5 puts write requests for about kern.geom.raid5.wdt seconds (but not less
>> than 1-2 seconds) into the write cache (if there is enough space left in
>> graid5's write cache)... I would guess that this behaviour is pretty
>> incompatible with soft-updates with power outage...
> 
> Can this cache be disabled?

Probably - but recent info shows it to be the prime mover in providing decent 
performance (when things are NOT broken).

> 
>> Then there still is the write cache of the hard discs (I dont know how long it
>> waits, but that time would come in addition to graid5's delay)...
>>
>> Maybe gjournal could help, because graid5 honors the BIO_FLUSH, but that is
>> untested...
> 
> Yes, AFAIK this would work.
> 

A RAID5 is one of the harder ones to do both fast and well in software-only.

The better hardware ($$$) controllers have fast hardware XOR engines as well as 
CPU-as-state-machines and battery-backed cache, and THEY have to work hard.

Further,  a hardware controller sits in the right place to do the job well, the 
'GP' CPU(s) - no matter they have spare cycles to burn - do not.

I don't think even GEOM magic can get around that w/o user willingness to take 
on some unavoidable compromises.

Given decent hardware & any UPS that costs less than the hardware controller, 
these are 'choices' - not really show-stoppers.

Bill


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