On tis, 2017-08-08 at 10:24 -0300, FERNANDO FREDIANI wrote:
> That's something on the way RAID works, regardless what most 
> 'super-ultra' powerfull hardware controller you may have. RAID 5 or
> 6 
> will never have the same write performance as a RAID 10 o 0 for
> example. 
> Writeback caches can deal with bursts well but they have a limit 
> therefore there will always be a penalty compared to what else you
> could 
> have.
> 
> If you have a continuous stream of data (a big VM deployment or a
> large 
> data copy) there will be a continuous write and that will likely fill
> up 
> the cache making the disks underneath the bottleneck.
> That's why on some other scenarios, like ZFS people have multiple
> groups 
> of RAID 6 (called RAIDZ2) so it improves the write speeds for these
> type 
> of scenarios.

Just pointing out that it is commonly known as RAID 60, outside of the
ZFS lingo:
https://en.wikipedia.org/wiki/Nested_RAID_levels#RAID_60

/K

> 
> In the scenario given in this thread with just 3 servers, each with
> a 
> RAID 6 there will be a bare limit on the write performance specially
> for 
> streammed data for most powerfull your hardware controller can do 
> write-back.
> 
> Also I agree the 40Gb NICs may not be used fully and 10Gb can do the
> job 
> well, but if they were available at the begining, why not use them.
> 
> Fernando
> 
> 
> On 08/08/2017 03:16, Fabrice Bacchella wrote:
> > 
> > > 
> > > Le 8 août 2017 à 04:08, FERNANDO FREDIANI <fernando.frediani@upx.
> > > com> a écrit :
> > > Even if you have a Hardware RAID Controller with Writeback cache
> > > you will have a significant performance penalty and may not fully
> > > use all the resources you mentioned you have.
> > > 
> > Nope again,from my experience with HP Smart Array and write back
> > cache, write, that goes in the cache, are even faster that read
> > that must goes to the disks. of course if the write are too fast
> > and to big, they will over overflow the cache. But on todays
> > controller they are multi-gigabyte cache, you must write a lot to
> > fill them. And if you can afford 40Gb card, you can afford decent
> > controller.
> > 
> > 
> > 
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