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From: west@sdcsla.UUCP (Larry West)
Newsgroups: net.unix-wizards
Subject: Re: UNIX 4.2 thrashing - the cause?
Message-ID: <766@sdcsla.UUCP>
Date: Wed, 23-Jan-85 06:17:24 EST
Article-I.D.: sdcsla.766
Posted: Wed Jan 23 06:17:24 1985
Date-Received: Fri, 25-Jan-85 21:23:59 EST
References: <1282@kaist.UUCP> <1608@ittvax.UUCP>
Reply-To: west@sdcsla.UUCP (Larry West)
Organization: UC San Diego: Institute for Cognitive Science
Lines: 39
Summary: 

In article <1608@ittvax.UUCP> long@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%.

-- 

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