UNIX 4.2 thrashing - the cause?

[Larry West]

In article <1608 at ittvax.UUCP> long at ittvax.UUCP (H. Morrow Long
[Systems Center]) writes: {this is severely edited!}:
 > > Somewhile ago, our VAX 780 showed a disastrous performance.
 > > As far as I can guess, it surely is a thrashing, a rarely occurence.
 > > Here is the disk usage for reference ( any hint? ).
 > > 
 > > Filesystem    kbytes    used   avail capacity  Mounted on
 > > /dev/hp0a       7421    6519     159    98%    /
 > > /dev/hp2h     137616  120775    3079    98%    /va <- user area
 > > /dev/hp0g      38639   34747      28   100%    /vb <- user area
 >
 >We have also experienced the problem described above.  Here is what I found
 >out:
 >	"In order for the layout policies to be effective, the disk cannot be
 >	kept completely full.  Each file system maintains a parameter that
 >	gives the minimum acceptable percentage of file system blocks that can
 >	be free. 
 >
 >I believe there is a lesson here.  4.2bsd sites should try to keep all
 >filesystems below 90% full (especially those where a great amount of creation
 >and deletion take place daily - /usr, /usr/spool) or suffer degradation.
 >

>From the 4.2bsd manual on "df" (which both previous contributors were
using to determine the "fullness" of their disks):

	Note that used+avail is less than the amount of space in the
	file  system (kbytes); this is because the system reserves a
	fraction of the space in the file system to allow  its  file
	system   allocation  routines  to  work  well.   The  amount
	reserved is typically about 10%; 

So a "df" listing saying 100% really means (ignoring "tunefs(8)") 90%.

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--|  Larry West, UC San Diego, Institute for Cognitive Science
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