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From: Poul-Henning Kamp <phk@FreeBSD.org>
Date: Wed, 10 Mar 2004 12:41:09 -0800 (PST)
To: src-committers@FreeBSD.org, cvs-src@FreeBSD.org,
	cvs-all@FreeBSD.org
X-FreeBSD-CVS-Branch: HEAD
Subject: cvs commit: src/sys/sys mdioctl.h src/sys/dev/md md.c
 src/sbin/mdconfig mdconfig.8 mdconfig.c
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phk         2004/03/10 12:41:09 PST

  FreeBSD src repository

  Modified files:
    sys/sys              mdioctl.h 
    sys/dev/md           md.c 
    sbin/mdconfig        mdconfig.8 mdconfig.c 
  Log:
  Fix a long-standing deadlock issue with vnode backed md(4) devices:
  
  On vnode backed md(4) devices over a certain, currently undetermined
  size relative to the buffer cache our "lemming-syncer" can provoke
  a buffer starvation which puts the md thread to sleep on wdrain.
  
  This generally tends to grind the entire system to a stop because the
  event that is supposed to wake up the thread will not happen until a fair
  bit of the piled up I/O requests in the system finish, and since a lot
  of those are on a md(4) vnode backed device which is currently waiting
  on wdrain until a fair amount of the piled up ... you get the picture.
  
  The cure is to issue all VOP_WRITES on the vnode backing the device
  with IO_SYNC.
  
  In addition to more closely emulating a real disk device with a
  non-lying write-cache, this makes the writes exempt from rate-limited
  (there to avoid starving the buffer cache) and consequently prevents
  the deadlock.
  
  Unfortunately performance takes a hit.
  
  Add "async" option to give people who know what they are doing the
  old behaviour.
  
  Revision  Changes    Path
  1.27      +3 -0      src/sbin/mdconfig/mdconfig.8
  1.31      +5 -1      src/sbin/mdconfig/mdconfig.c
  1.115     +5 -3      src/sys/dev/md/md.c
  1.14      +1 -0      src/sys/sys/mdioctl.h