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It simple - use SSD. only. OK. The price difference between 15k SAS and SSD
is negative (15k is more costly), and differce between 10k SAS and SSD is
small. SSD is pricier than a low end SATA, but I don't want to be near
system running instances on SATA array.

So answer simple - SSD. Use 5/6 RAID to save a bit money in exchange for
write performance loss, but it still gonna be much faster than spindles.
On May 8, 2014 3:24 PM, "Arne Wiebalck" <Arne.Wiebalck at cern.ch> wrote:

>  Hi all,
>
>  In our cloud we use the non-shared local fs of the compute for instance
> storage. As our cloud gets more busy, this is now more and more becoming a
> serious bottleneck.
>
>  We saw that tuning the configuration, such as the IO scheduler, can
> improve things significantly, but in the end things are of course limited
> by the h/w used in the
> hypervisors (which are RAIDed spinning disks in our case). For now, we
> don't want to go down the roads of local shared storage (due the additional
> complexity to
> set this up) nor the road of non-local shared storage (to continue to
> profit from the limited impact of storage failures). So we are considering
> to add SSDs to our hardware
> setup, which can then either be used directly or via a block level caching
> mechanism (e.g. bcache).
>
>  When discussing the various options to set this up, we wondering how
> other clouds deal with the problem of compute disk contention in general
> and the integration
> of SSDs in particular.
>
>  So, any suggestions or experiences in this area you'd like to share
> would be very welcome!
>
>  Thanks!
>  Arne
>
>    --
>  Arne Wiebalck
> CERN IT
>
>
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>
>
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