linux-kernel - Re: [ANNOUNCE] Ramback: faster than a speeding bullet

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Message-ID: <x498x0lb3ad.fsf@segfault.boston.devel.redhat.com>
Date:	Fri, 14 Mar 2008 13:53:30 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	David Newall <davidn@...idnewall.com>, david@...g.hm,
	Chris Friesen <cfriesen@...tel.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet

Daniel Phillips <phillips@...nq.net> writes:

> 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.

Ramback could be an interesting building block.  Consider using a
couple of systems exporting Ramback devices via Evgeniy's distributed
storage target (or something similiar).  In this case, you can have as
many Ramback devices as you want comprise your mirror set to meet your
availability requirements.  Perhaps people are looking at this too
much as an entire solution as opposed to a piece of a bigger puzzle.
I think the idea has merit.

Cheers,

Jeff
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