Re: CentOS 6.0 and 3ware 9650SE series RAID Performance

[Austin Godber <godber@xxxxxxxxx>]

Thank you for the clarification Craig, I am re-examining my storage strategy, thus my email.  Sadly this machine has been in service a number of years and already contains more data than is possible in a RAID 10 configuration.  If I had the spare space and resources I'd be thrilled to switch to RAID 10.  That is not the case, however.  Given that, I am faced with the choice between having the machine entirely unusable between now and when I can acquire a BBU or using it with the risk of data loss/corruption, I will chose the latter.  Neither data loss nor corruption would go unnoticed in the period that this will be necessary.

Does that sound entirely unreasonable?

Austin

On Fri, Sep 2, 2011 at 10:51 AM, Craig White <craig.white@xxxxxxxxxx> wrote:

Data loss could conceivably occur on shutdown or restart too - just saying... You are assuming that the data that doesn't get written to disk is going to be non-essential... I wish you good luck with that. I think if one doesn't want to be an idiot, one would not enable a cache that has no means to ensure that the cache is written to disk.

I think your take away from all of this is somewhat misdirected. Not having a BBU simply means that your writes really should always be synchronous/immediate. That shouldn't really be a problem and shouldn't impose a large performance penalty.

Your performance issue relates more to the fact that RAID 5 implementation on the 3Ware cards is rather poor and modes such as RAID 10 (RAID 0 + 1) will give you much more speed that you realize. If you also consider on the surprisingly higher rates of failure with loss of data possibility when reconstructing a missing/dead drive on a RAID 5 setup you really should be re-examining your storage strategy.

Craig

On Sep 1, 2011, at 5:43 PM, Austin Godber wrote:

> At this point the card is pretty much useless without that cache enabled.  Without recommendations for making writes of 256MB or larger files faster without this cache enabled, I will have to accept the possible data loss in the event of power outage.  If it is only the case of data loss during a power outage, I will take that ... rather than failure to write at all during 99% of my usage.
>
> I will, for the sake of not being an idiot, look into buying the BBUs.
>
> Austin
>
> On Thu, Sep 1, 2011 at 4:49 PM, Tom Bishop <bishoptf@xxxxxxxxx> wrote:
> Keep in mind you really only want to enable the cache if you have a
> bbc, otherwise you are risking your data since it can/will cache
> writes...just something to keep in mind.
>
> On 9/1/11, Austin Godber <godber@xxxxxxxxx> wrote:
> > Hi Craig,
> >
> > Thanks for the suggestion.  I would if I could.  I'd also probably try
> > another file system.  Though the good news is, enabling the write cache on
> > that array has improved things significantly.  Which, in my case, was:
> >
> > tw_cli /c2/u0 set cache=on
> >
> > Now, if only I had the battery backup unit for the card.
> >
> > Thanks, everyone for their suggestions.  For now I am happy with the
> > situation, but I'd be interested to hear the experiences of others.
> >
> > Austin
> >
> > On Thu, Sep 1, 2011 at 3:58 PM, Craig White <craig.white@xxxxxxxxxx> wrote:
> >
> >>
> >> On Sep 1, 2011, at 1:41 PM, Austin Godber wrote:
> >>
> >> > Hello,
> >> >
> >> > Does anyone have experience using a 3ware 9650SE series raid controller
> >> on CentOS 6.0?
> >> ----
> >> use RAID 10
> >>
> >> Unless something has changed, RAID 5 is notoriously slow on the 3Ware
> >> controllers. Whatever you do will only incrementally speed things up. If
> >> performance is desired, RAID 5 is not the way to go.
> >>
> >> Craig
> >> _______________________________________________
> >> CentOS mailing list
> >> CentOS@xxxxxxxxxx
> >> http://lists.centos.org/mailman/listinfo/centos
> >>
> >
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