From: Neil Brown <neilb@cse.unsw.edu.au>
To: Wilco Baan Hofman <wilco@baanhofman.nl>
Date: Mon, 5 Sep 2005 11:27:55 +1000
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Message-ID: <17179.40731.907114.194935@cse.unsw.edu.au>
Cc: linux-kernel@vger.kernel.org
Subject: Re: RAID1 ramdisk patch
In-Reply-To: message from Wilco Baan Hofman on Monday September 5
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On Monday September 5, wilco@baanhofman.nl wrote:
> Hi all,
> 
> I have written a small patch for use with a HDD-backed ramdisk in the md 
> raid1 driver. The raid1 driver usually does read balancing on the disks, 
> but I feel that if it encounters a single ram disk in the array that 
> should be the preferred read disk. The application of this would be for 
> example a 2GB ram disk in raid1 with a 2GB partition, where the ram disk 
> is used for reading and both 'disks' used for writing.
> 
> Attached is a bit of code which checks for a ram-disk and sets it as 
> preferred disk. It also checks if the ram disk is in sync before 
> allowing the read.

Hi,
 equivalent functionality is now available in 2.6-mm and is referred
 to as 'write mostly'.
 If you use mdadm-2.0 and mark a device as --write-mostly, then all
 read requests will go to the other device(s) if possible,.
 e.g.
   mdadm --create /dev/md0 --level=1 --raid-disks=2 /dev/ramdisk \
      --writemostly /dev/realdisk

 Does this suit your needs?

 You can also arrange for the write to the writemostly device to be
 'write-behind' so that the filesystem doesn't wait for the write to
 complete.  This can reduce write-latency (though not increase write
 throughput) at a very small cost of reliability (if the RAM dies, the
 disk may not be 100% up-to-date).

NeilBrown

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