reproduced: ffs_blkfree: freeing free block

[Poul-Henning Kamp]

Quite by accident my test-machine here can now reliably reproduce
the dreaded "panic: ffs_blkfree: freeing free block" in a few minutes
of time.

It is very interesting that the location of the actual error is a
very narrow stripe of the filesystem:

	dev = ad8, block = 13456368, fs = /hex
	dev = ad8, block = 13455888, fs = /hex
	dev = ad8, block = 13454688, fs = /hex
	dev = ad8, block = 13455040, fs = /hex
	dev = ad8, block = 13455200, fs = /hex
	dev = ad8, block = 13455880, fs = /hex

The application I'm running at the time adds/modifies records in a
db(3) hash file and nothing much besides.

Before I start implementing complete I/O traces and spend days
groveling over UFS/FFS on-disk bits, are there anybody who has
suggestions for things I should try to enable/disable to narrow
this down ?

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.