Re: 64-bit block sizes on 32-bit systems

Andreas Dilger
2001-03-26T17:48:43Z

Matthew Wilcox writes:
> On Mon, Mar 26, 2001 at 08:39:21AM -0800, LA Walsh wrote:
> > I vaguely remember a discussion about this a few months back.
> > If I remember, the reasoning was it would unnecessarily slow
> > down smaller systems that would never have block devices in
> > the 4-28T range attached.
>
> 4k page size * 2GB = 8TB.
>
> i consider it much more likely on such systems that the page size will
> be increased to maybe 16 or 64k which would give us 32TB or 128TB.
>
> personally, i'm going to see what the situation looks like in 5 years time
> and try to solve the problem then.

What do you mean by problems 5 years down the road? The real issue is that
this 32-bit block count limit affects composite devices like MD RAID and
LVM today, not just individual disks. There have been several postings
I have seen with people having a problem _today_ with a 2TB limit on
devices.

There is some hope with LVM (and MD I suspect as well), that it could
do blocksize remapping, so it appears to be a 4k sector device, but
remaps to 512-byte sector disks underneath. This _should_ give us an
upper limit of 16TB, assuming 32-bit unsigned ints for block numbers.
Of course, you would need to only do 4kB block I/O on top of these devices
(not much of an issue for such large devices).

Still, this is just a stop-gap measure because next year people will want
> 16TB devices, and there won't be an easy way to do this.

Cheers, Andreas
--
Andreas Dilger \ "If a man ate a pound of pasta and a pound of antipasto,
\ would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/ -- Dogbert