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

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Date:	Mon, 10 Mar 2008 09:22:13 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [ANNOUNCE] Ramback: faster than a speeding bullet

> So now you can ask some hard questions: what if the power goes out 
> completely or the host crashes or something else goes wrong while 
> critical data is still in the ramdisk?  Easy: use reliable components.  

Nice fiction - stuff crashes eventually - not that this isn't useful. For
a long time simply loading a 2-3GB Ramdisk off hard disk has been a good
way to build things like compile engines where loss of state is not bad.

> If UPS power runs out while ramback still holds unflushed dirty data 
> then things get ugly.  Hopefully a fsck -f will be able to pull 
> something useful out of the mess.  (This is where you might want to be 
> running Ext3.)  The name of the game is to install sufficient UPS power 
> to get your dirty ramdisk data onto stable storage this time, every 
> time.

Ext3 is only going to help you if the ramdisk writeback respects barriers
and ordering rules ?

>  * Previously saved data must be reloaded into the ramdisk on startup.

/bin/cp from initrd

>  * Cannot transfer directly between ramdisk and backing store, so must 
>    first transfer into memory then relaunch to destination.

Why not - providing you clear the dirty bit before the write and you
check it again after ? And on the disk size as you are going to have to
suck all the content back in presumably a log structure is not a big
concern ?

>  * Per chunk locking is not feasible for a terabyte scale ramdisk.

And we care  8) ?

>   * Handle chunk size other than PAGE_SIZE.

If you are prepared to go bigger than the fs chunk size so lose the
ordering guarantees your chunk size really ought to be *big* IMHO

Alan
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