[DRBD-user] drbd on top of hardware raid

Matthew Wilson matthew at sixapart.com
Wed Aug 9 23:37:20 CEST 2006

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


On Aug 9, 2006, at 1:27 PM, Lars Ellenberg wrote:

> / 2006-08-09 13:53:39 -0600
> and, btw, you probably know that already ...
> raid controllers tend to pretend against the kernel that data was
> on stable storage, even when they indeed only have it still in their
> write back cache.
> if that cache is battery backed, ok.
> if not, and you get data corruption after blackout/brownout,
> don't blame linux, nor the device/file system drivers,
> nor drbd either...
>
> unfortunately sometimes you can not put it in "write through" mode
> (use that cache, but only say its done when its really done), but
> only switch off the cache completely.
> and then your write throughput gets really bad...
>
> so you better make sure you run on some ups, and monitor it,
> and power down the box cleanly in case the ups capacity runs out.

Drive manufacturers also like to enable the write caches on their  
drives.  It's the WCE bit in the disk mode pages.  Unfortunately it's  
not possible to modify this bit through a megaraid.  There's a flag  
to temporarily disable it at runtime but rebooting or resetting a  
disk will once again set it to write cache.  My preferred method is  
to yank the drives and use sdparm (http://sg.torque.net/sg/ 
sdparm.html) through a plain ol scsi controller to write the mode pages.

If you want to test that your disks are behaving nicely or badly you  
can use the script referenced here:

http://brad.livejournal.com/2116715.html

I've run that against drbd with a good underlying store and found  
drbd to work perfectly with respect to power loss situations.




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