On Sat, Sep 21, 2013 at 09:58:30AM +0000, Michael van Elst wrote: > [email protected] (Edgar =?iso-8859-1?B?RnXf?=) writes: > > >I myself can't make sense out of the combination of > >-- vfs.wapbl.flush_disk_cache=0 mitigating the problem > >-- neither the RAID set nor its components showing busy in iostat > >Maybe during a flush, the discs are not regarded busy? > > busy means that the driver is executing I/O requests or waiting for > results from such operartions. The cache flush operation doesn't > count by itself, but it slows down parallel I/O operations, so the > disk appears busy as long as there are such I/O operations.
At last for wd(4) (and I suspect it's true for sd(4) as well), new I/Os while a flush cache is running are stalled in the buffer queue, and so disk_busy() is only called once the flush cache has completed. So, the disk is effectively counted as unbusy while a cache flush is in progress. -- Manuel Bouyer <[email protected]> NetBSD: 26 ans d'experience feront toujours la difference --
