Re: [PATCH 00/14] GFS

[David Teigland <teigland@redhat.com>]

On Fri, Aug 05, 2005 at 09:34:38AM +0200, Arjan van de Ven wrote:
> On Fri, 2005-08-05 at 15:14 +0800, David Teigland wrote:
> > On Tue, Aug 02, 2005 at 09:45:24AM +0200, Arjan van de Ven wrote:
> > 
> > > * +static const uint32_t crc_32_tab[] = .....
> > > why do you duplicate this? The kernel has a perfectly good set of
> > > generic crc32 tables/functions just fine
> > 
> > The gfs2_disk_hash() function and the crc table on which it's based are a
> > part of gfs2_ondisk.h: the ondisk metadata specification.  This is a bit
> > unusual since gfs uses a hash table on-disk for its directory structure.
> > This header, including the hash function/table, must be included by user
> > space programs like fsck that want to decipher a fs, and any change to the
> > function or table would effectively make the fs corrupted.  Because of
> > this I think it's best for gfs to keep it's own copy as part of its ondisk
> > format spec.
> 
> for userspace there's libcrc32 as well. If it's *the* bog standard crc32
> I don't see a reason why your "spec" can't just reference that instead.
> And esp in the kernel you should just use the in kernel one not your own
> regardless; you can assume the in kernel one is optimized and it also
> keeps size down.

linux/lib/crc32table.h : crc32table_le[] is the same as our crc_32_tab[].
This looks like a standard that's not going to change, as you've said, so
including crc32table.h and getting rid of our own table would work fine.

Do we go a step beyond this and use say the crc32() function from
linux/crc32.h?  Is this _function_ as standard and unchanging as the table
of crcs?  In my tests it doesn't produce the same results as our
gfs2_disk_hash() function, even with both using the same crc table.  I
don't mind adopting a new function and just writing a user space
equivalent for the tools if it's a fixed standard.

Dave

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