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List:       netbsd-tech-kern
Subject:    Re: SCSI MMC device abstraction and UDF patch for review
From:       Bill Studenmund <wrstuden () NetBSD ! org>
Date:       2005-12-28 21:33:34
Message-ID: 20051228213334.GA21625 () netbsd ! org
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On Sat, Dec 24, 2005 at 02:41:54PM +0100, Reinoud Zandijk wrote:
> Hiya Takashi,
> 
> On Sat, Dec 24, 2005 at 08:35:22PM +0900, YAMAMOTO Takashi wrote:
> > > ftp://ftp.NetBSD.org/pub/NetBSD/misc/reinoud/src-diffs-udf.20051222.gz
> > 
> > - it seems to do double-caching.  ie. at both of udf and block device.
> >   is it intended?
> 
> Yes and no. For UDF it is better to use caching in udf since blocks can 
> move around esp. on recordables; every disc write changes the location of 
> the block so its easier to have it cached on the file's vnode on its 
> logical block instead of a moving buffer on disc. Setting B_NOCACHE in the 
> buffers passed to the block device is prolly advisable yes.

Why is it easier?

UDF is not the first file system in our kernel to face this issue. LFS has 
been dealing with it for over a decade. I think it would be VERY advisable 
for UDF to do the exact same thing for a number of reasons:

1) LFS has worked out a huge number of the issues, so UDF can gain from 
that experience.

2) It will be much easier for others to maintain the code if we only have 
one way that we cope with block-shuffling file systems.

3) If UDF shows us we really need to fix an issue, LFS may gain from the 
same changes.

Take care,

Bill

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