Re: RAID1 ramdisk patch

[Wilco Baan Hofman <wilco@baanhofman.nl>]

Neil Brown wrote:

> 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
>
>  
I was looking for that (but couldn't find it)..

At this point I don't see why it wouldn't, if that also syncs from the 
partition then it's basically the same functionality, but written from a 
different perspective.

To use it I'll have to deviate from stock linux and use a non-packaged 
mdadm, but that is better than applying my patch every kernel update ;-)

Thanks, I'll look into it.

Wilco Baan Hofman
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