Re: RAID1 ramdisk patch
From: Wilco Baan Hofman
Date: Mon Sep 05 2005 - 02:40:59 EST
Neil Brown wrote:
On Monday September 5, wilco@xxxxxxxxxxxxx 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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