Anyone establish optimum blockdev --setra settings for raid on a 2.6 kernel?
There has been some discussions on the lvm mailing list.
In the case of lvm on raid sounds like it's best to use 0 on the md and disk devices and something around 1024 and 4096 on the lvm devices.
It seems make some sense that it can cause a lot of unneeded reads particularly if you have a lot of layers and/or a lot of raid5 drives.
My experiments have been inconclusive. At least it seems that setting read ahead to 0 on the low level devices has no penalty.
John
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