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

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Date:	Sat, 15 Mar 2008 16:40:22 -0400
From:	"Mike Snitzer" <snitzer@...il.com>
To:	"Pavel Machek" <pavel@....cz>
Cc:	"Daniel Phillips" <phillips@...nq.net>,
	"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

On Sat, Mar 15, 2008 at 4:26 PM, Pavel Machek <pavel@....cz> wrote:
> On Thu 2008-03-13 12:03:03, Daniel Phillips wrote:
>  > 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.
>
>  raid1 + kflushd tweak?
>
>  special raid1 mode that signals completion when it hits _one_ of the
>  drives, and does sync when the slower drive is idle?

raid1 already supports marking member(s) as write-mostly.  Any
write-mostly member can also make use of write-behind mode (provided
you have a write intent bitmap).
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