Messages in this thread | | | From | Neil Brown <> | Date | Mon, 5 Sep 2005 11:27:55 +1000 | Subject | Re: RAID1 ramdisk patch |
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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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