From: Daniel Phillips <phillips@phunq.net>
To: David Newall <davidn@davidnewall.com>
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet
Date: Thu, 13 Mar 2008 12:03:03 -0800
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Cc: david@lang.hm, Chris Friesen <cfriesen@nortel.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org
References: <200803092346.17556.phillips@phunq.net> <200803131232.07879.phillips@phunq.net> <47D9856E.9010304@davidnewall.com>
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On Thursday 13 March 2008 12:50, David Newall wrote:
> Daniel Phillips wrote:
> > The period where you cannot access the data is downtime.  If your script
> > just does a cp from a disk array to the ram device you cannot just read
> > from the backing store in that period because you will need to fail over
> > to the ramdisk at some point, and you cannot just read from the ramdisk
> > because it is not populated yet.
> 
> Wouldn't a raid-1 set comprising disk + ramdisk do that with no downtime?

In raid1, write completion has to wait for write completion on all
mirror members, so writes run at disk speed.  Reads run at ramdisk
speed, so your proposal sounds useful, but ramback aims for high
write performance as well.

Daniel
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