[ale] Onboard RAID

[Michael B. Trausch]

On 11/17/2011 11:58 AM, Brian Mathis wrote:
> In addition, the new PERCs support battery RAM backup to flash RAM,
> giving you 10 years to get the data back after a power failure.

I don't follow.  Why would the thing need to hold RAM for a decade?

Any RAID unit should be backed by a UPS that will outlast the UPS used
to keep the server up and running, and ideally should have a good enough
internal battery to make sure that in the event of a PF event, all
presently cached writes make it to the platters.  "Stupid" boxes (that
is, ones that do not have RAM) are simpler in this regard because all
they have to do is wait for the HDDs to flush their cache.  Smarter
boxes will flush the RAM buffers to the HDD buffers and instruct the HDD
to flush its buffers to the platter.

It sounds to me like the type of feature that exists only as a component
of some sort of pissing contest.

> They
> also support SSD caching, so if you add an SSD to an array of regular
> disks, the most actively read sectors are cached on the SSD.

Linux can do that, too.  (I haven't dropped a setup with that capability
in production, because I do not have hard-core performance
requirements... but it's doable with the kernel's [relatively] new
generic disk-caching framework.)  Shoot, even Windows can do that.

	--- Mike

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