Re: choosing RAID level for xlogs

Don't forget that often controlers don't obey fsyncs like a plain
drive does.  thats the point of having a BBU ;)

Alex Turner
NetEconomist

On 8/16/05, John A Meinel  wrote:
> Anjan Dave wrote:
> > Yes, that's true, though, I am a bit confused because the Clariion array
> > document I am reading talks about how the write cache can eliminate the
> > RAID5 Write Penalty for sequential and large IOs...resulting in better
> > sequential write performance than RAID10.
> >
> > anjan
> >
> 
> To give a shorter statement after my long one...
> If you have enough cache that the controller can write out big chunks to
> the disk at a time, you can get very good sequential RAID5 performance,
> because the stripe size is large (so it can do a parallel write to all
> disks).
> 
> But for small chunk writes, you suffer the penalty of the read before
> write, and possible multi-disk read (depends on what is in cache).
> 
> RAID10 generally handles small writes better, and I would guess that
> 4disks would perform almost identically to 6disks, since you aren't
> usually writing enough data to span multiple stripes.
> 
> If your battery-backed cache is big enough that you don't fill it, they
> probably perform about the same (superfast) since the cache hides the
> latency of the disks.
> 
> If you start filling up your cache, RAID5 probably can do better because
> of the parallelization.
> 
> But small writes followed by an fsync do favor RAID10 over RAID5.
> 
> John
> =:->
> 
> 
>