Date: Thu, 25 Jun 1998 00:19:58 +1000 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, peter@netplex.com.au Cc: chuckr@glue.umd.edu, freebsd-current@FreeBSD.ORG, phk@critter.freebsd.dk Subject: Re: Heads up: block devices to disappear! Message-ID: <199806241419.AAA19246@godzilla.zeta.org.au>
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>> Block devices are also used for buffering. Examples:
>> a) Linux fsck.ext2 does partial sector i/o's, so it does not work
>> on character disk devices. It works OK on block disk devices
>> despite lack of support for some special ioctls.
>> b) The simple binary editor that I use reads/writes precisely the
>> bytes bytes inspected/changed. This is normally the correct thing
>> for a binary editor to do, but it does not work on character disk
>> devices.
>
>Both of these should be able to be made to work for character access to
>block orientated devices. specfs truncates and rounds the block accesses
>for VBLK devices, I don't see why physio couldn't do something similar.
Truncate/round on works for disk-like block devices. It didn't and
couldn't work for tape-like devices. I don't like doing it automatically
because using an unusual block size is usually an error.
>Incidently, in kern_physio.c:
>
>struct buf *
>phygetvpbuf(dev_t dev, int resid)
>{
> struct bdevsw *bdsw;
> int maxio;
>
> bdsw = cdevsw[major(dev)]->d_bdev;
> if (bdsw == NULL)
> return getpbuf();
>
> maxio = bdsw->d_maxio;
> if (resid > maxio)
> resid = maxio;
>
> return getpbuf();
>}
>
>What is the point of passing in resid and calculating maxio? It's not
>used... the calls to phygetvpbuf() could just as easily be calls to
>getpbuf() directly..
It seems to be unfinished code. My version of it has always been:
struct buf *
physgetvpbuf(dev, resid)
dev_t dev;
int resid;
{
#ifdef NONSENSE
struct bdevsw *bdsw;
int maxio;
bdsw = cdevsw[major(dev)]->d_bdev;
if (bdsw == NULL)
return (getpbuf());
maxio = bdsw->d_maxio;
if (resid > maxio)
resid = maxio;
#endif
return (getpbuf());
}
>> c) Caching of (small) slow media, e.g.,:
>> sleep 1000 </dev/fd0 # prevent last close
>> tar tvvf /dev/fd0 # inspect floppy contents
>> tar xvvf /dev/fd0 ... # read floppy contents from buffer cache.
>> The last close prevention may be unnecessary under Linux (Linux at
>> least used to not invalidate the cache until disk change), and doesn't
>> actually work under FreeBSD (a comment in spec_close() says that
>>...
>tar should be able to lseek() in cases like this (like it does on SCO for
>example) meaning this should be largely irrelevant.
Not for small (< 8K) files in the tar image. Then read-ahead on the
buffered should result in the entire image being read in the same time
as it takes to lseek and read tar headers.
>The specfs last close support is a mess. specfs itself is a hack that
>exists because of the need to glue vnodes onto the devsw. If devsw
>interface was replaced, driver interface could be vnode oriented. Anyway,
Then last vnode dereference support would be a mess :-).
Bruce
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