Re: blocks or KB? (was: .. current meaning of blk_size array)

[Andreas Dilger <adilger@turbolabs.com>]

On Nov 14, 2001  21:41 +0100, Peter T. Breuer wrote:
>   "A month of sundays ago Martin Dalecki wrote:"
>   > "Peter T. Breuer" wrote:
>   > > Is blk_size[][] supposed to contain the size in KB or blocks?
>   > There is no rumor it's in blocks.
> 
> Maybe I misinterpret what you write. I interpret it as meaning "the
> rumour is not a rumour but a fact. It is in blocks".

Check what /proc/partitions shows us.  #blocks, with units of 1kB.
This has been standard in the kernel for a loooong time.

> The point is that if blk_size counts in KB, then the size of a device
> cannot reach more that 2^32 * 2^10 = 2^42 = 4TB.  I'd personally say
> 2TB, becuase the int blk_size number is signed.
> 
> That's rumoured not to be the case, and the max size of a device is
> supposed to be about 8 to 16TB. Let's suppose the rumour is true ..

Well, the rumor is wrong.  There has always been a single-device 1TB/2TB
limit in the kernel (2^31 or 2^32 * 512 byte sector size), and until
recently it has not been a problem.  To remove the problem Jens Axboe
(I think, or Ben LaHaise, can't remember) has a patch to support 64-bit
block counts and has been tested with > 2TB devices.

> So we deduce that one has to assign a different meaning for the blk_size
> array. "count in blocks" is how the rumour goes. That way you can get
> 4 times higher sizes .. all the way to 8 or 16TB per device. And this
> is what is rumoured to be the case.

Where do you get these rumors?

> It should be in blocks if the size of a device is to reach 8 or 16TB.
> If it is in KB, we are limited to 2 or 4TB.

In theory this is possible (it was discussed on the LVM list a bit), but
it would take a bunch of work to make it real.  For LVM (and MD RAID),
since we are dealing with multiple real devices < 2TB in size, we could
use a blocksize of 4kB to get a larger virtual device.  In the end this
only wins for a short time and you need 64-bit block numbers anyways.

> Persuade me that this is not a bug, and an important one at that :-)
> Hellloooooo everybody! Linux cannot manage partitions greater than
> 4TB, ha ha ha hhhhhaaaa! ;-)

And it can't handle more than 64GB of RAM on ia32 (was previously 1GB).
So what?  When a limit is reached for any reasonable number of people,
it is fixed.

> I at least am getting up to devicesizes at the 8TB range.

If you are in that ballpark, then get the 64-bit blocknumber patch, and
start testing/fixing, instead of complaining.

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/