DM multipath limits IO size to 512kB

Neil,

In Linux you can change the maximum IO size of a block device from /sys/block/sd*/queue/max_sectors_kb. It can be any size up to max_hw_sectors_kb.

When DM multipath is used to manage failover between multiple links to a SAN, the virtual device's max_sectors_kb variable cannot be changed from sysfs. Moreover, by default, multipath sets max_sectors_kb of the virtual device to the minimum between max_sectors_kb of each devices and 512kB.

The attached patch changed that behavior to the minimum between max_sectors_kb of each devices.

This patch is for the SLES10sp1 kernel (2.6.16.46-0.12); but should work on 2.6.27.7. We've been using it for a few months without any issue.

Note: when you attach to the LUN, the default value for max_sectors_kb is 512. If multipathd is running at that time, it will create the virtual device with max_sectors_kb set to 512. In order to make it work, you have to shutdown multipathd, attach to the LUN, change max_sectors_kb for each path and then start multipathd.

If you need anymore information, let me know.

I submitted this as an enhancement to the lustre filesystem: https://bugzilla.lustre.org/show_bug.cgi?id=17798

Thanks,

Julien

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