[DRBD-user] Performance Issue - HP cciss

Ben Clewett ben at roadrunner.uk.com
Wed Jan 30 14:19:55 CET 2008

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

Lars Ellenberg wrote:
 > On Wed, Jan 30, 2008 at 11:12:54AM +0000, Ben Clewett wrote:
 >> Hi Lars,
 >> I am still having performance issues, ref DRBD locking.  You were 
kind enough to spend some time to worked out it was probably my 
underlying storage at fault.
 >> I am using HP cciss P400 hardware RAID controllers.  I have these in 
12 HP Proliant DL G5 servers, only four of these are on DRBD.  These 
four are the ones which suffer the problem.
 >> I have recently changed the read-ahead-buffer from the default 4 MB 
to 512 KB.  Therefore when the disk reads 1 or more bytes, it used to 
read another 4 MB into cache, and now reads 512 KB.  I can't take these 
servers off line to rigorously test, but anecdotally this seems to have 
made a difference for the better.
 >> I note my meta-disks are covered by this read-ahead-buffer.
 >> Do you think this might effect DRBD in some scenario?
 >> If you had to suggest a read-ahead-buffer size for the meta-disk, 
can you suggest what might be optimum?
 > since DRBD meta-data access most of the time is sinlge sector
 > synchronous IO, read-ahead is completely irrelevant here.

Oh well, back to the drawing board.

I think changing the read-ahead value might have caused the disk 
controller to re-start, therefore unlocking what ever was stuck on the 
card.  In which case I could just change this value from 'cron' every 
hour and never see this problem again :)

 >> Thanks again for your excellent analysis,
 > we also do support and consulting, you know :)

It may come to that, and I am happy to contract you if need be.  However 
I still have a few options, and I would like to fix it my self.  I have 
to maintain these objects of delight, and this is a good way of learning 
how they work, or not work :)

I'd like to ask the group if any owners of HP cciss disk controllers 
have experienced similar locking, evident by 100% 'iowait' on both sides 
of a DRBD connection.  If so, can they mail be the version they have and 
how they fixed it please?




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