On Monday September 5, [email protected] 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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