Delayed writes on removed files

[Root Boy Jim]

   From: Amos Shapir <amos at taux01.uucp>

   Here is something new  for a change: When a file  is removed, its blocks
   are released to  the free list. If  any writes were done,  some of these
   blocks may  still be in  the buffer pool,  waiting for a  delayed write.
   When such  buffers are reallocated,  they will  be written to  the disk,
   even though their contents are no longer relevant.

I find this hard to believe. If there *are* blocks waiting to be read,
they must be associated with an inode, which is in core, and is therefore
open by someone. If the file is removed, the link count in core will
still be (at least) one, and I/O should happen normally, even tho the
file has no directory entrys.

   The systems I have checked - 4.2  and sysV.3 - make no effort to prevent
   this. It seems  that in an environment in which  many files are created,
   written to, and then discarded (e.g. compilation) preventing unnecessary
   writes  to  disk  may  have  a  significant  effect  on  overall  system
   performance. Does anyone know of a system that does this, and how?

When the file *is* finally closed, the inode ref count becomes zero,
and the system should just ignore the delayed write flag and move the
blocks to the free list. Seems simple enuf, but I have not read the
code to check if it in fact does this.

Perhaps it isn't even necessary, as blocks are only allocated on
writes, which then overwrite the old data. I can't imagine that
the old sata would be written first, but truth is stranger than fiction.

Chris, say it ain't so!

	   Amos Shapir			(My other cpu is a NS32532)
   National Semiconductor (Israel)
   6 Maskit st. P.O.B. 3007, Herzlia 46104, Israel  Tel. +972 52 522261
   amos%taux01 at nsc.com  34 48 E / 32 10 N

	(Root Boy) Jim Cottrell	<rbj at icst-cmr.arpa>
	National Bureau of Standards
	Flamer's Hotline: (301) 975-5688
	The opinions expressed are solely my own
	and do not reflect NBS policy or agreement
..  If I had heart failure right now, I couldn't be a more fortunate man!!