Re: Possible optimization in ext2_file_write()

Re: Possible optimization in ext2_file_write()

[Stephen C. Tweedie]

Hi,

On Thu, 18 Mar 1999 23:46:57 +0530 (IST), V Ganesh
<ganesh@vxindia.veritas.com> said:

> 	it looks like whenever we write a partial block which 
> doesn't exist in the buffer cache, ext2_file_write() (and
> possibly the write functions of other filesystems) directly
> reads that block from the block device without checking if
> it is present in the page cache. 

Correct...

> Of course, typical UNIX programs/shell jobs don't indulge in
> this kind of behaviour. General workstation usage (X,
> kernel compiles etc.) for a day caused only 32 unnecessary
> reads. 

... and also correct.

> So unless there are any specific application categories which
> require this I guess it's not worth the trouble to patch.

I'd agree (strongly).  It ties in with your next question:

> Anyone working on a VM revamp or buffer/page cache unification ?

Yes.  We still need the buffer cache (or something very like it) for
filesystem metadata caching and for block IO.  However, 2.3 _will_ see
us using the page cache for data writeback (and we already have
prototype patches to support that sort of behaviour).  The linux-mm
list has been discussing it for some time.

--Stephen
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Re: Possible optimization in ext2_file_write()

[Manfred Spraul]

"Stephen C. Tweedie" wrote:
> 
> Hi,
> 
> On Thu, 18 Mar 1999 23:46:57 +0530 (IST), V Ganesh
> <ganesh@vxindia.veritas.com> said:
> 
> >       it looks like whenever we write a partial block which
> > doesn't exist in the buffer cache, ext2_file_write() (and
> > possibly the write functions of other filesystems) directly
> > reads that block from the block device without checking if
> > it is present in the page cache.
> 
> Correct...

I don't know what you are exactly talking about, but there is another
problem except speed:
Most modern harddisks remap bad sectors, so sometimes you can't read a
sector, but if you write the sector is remapped.

I.e. if you "create a new file, write 400 bytes, close the file, sync",
then the data sector should not be read.

Our current Windows 95 & Windows NT file system drivers read the data
sector, and that has caused problems (older ZIP disks, SyQuest,
my own damnaged harddisk?-I don't remember the details).

Regards,
	Manfred
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