Ext3 users - Jul 2001 - ext3 and 64 bit addressing

Dear List People:

I am doing some comparisons of different fs's and need to know where ext3 
stands in terms of 64 bit addressing. XFS and JFS claim to be full 64 bit 
systems (though are there not still kernel limitations that would, for the 
time being, reduce the ability to use a full 64 bits?), what about ext3??

TIA

iain
-- 
==Linux and open source do threaten Microsoft at the server operating 
system level. But even more dangerous, the open source model threatens 
the core business strategy of Microsoft - domination, monopoly, total 
control and restricted consumer choices. History has shown that hegemonists 
and those who restrict freedom ultimately fail. Microsoft is on the wrong 
side of history.
                                 -- Matt Szulik, CEO Red Hat Inc
==Iain Campbell, Senior Associate, sandon associates
520 Second Street West Owen Sound ON Canada N4K 6G6
Tel 519 371 6242 email iain@sandon.ca

On Fri, Jul 06, 2001 at 11:44:51PM -0400, Iain Campbell
wrote:
> Dear List People:
> 
> I am doing some comparisons of different fs's and need to know where
ext3
> stands in terms of 64 bit addressing. XFS and JFS claim to be full 64 bit 
> systems (though are there not still kernel limitations that would, for the 
> time being, reduce the ability to use a full 64 bits?), what about ext3??
> 
On a per-file basis, ext3 has a 41 bit file-size (number of blocks
used) limitation, caused by the fact that i_blocks in the inode is a
32 bit value which is addressed in 512 byte blocks.  This isn't a big
deal at the moment, given that limitations in the linux buffer cache
limit have the same limitation on the total filesystem size, at least
on 32-bit systems.  Fixing this limitation is pretty trivial kernel
patch and minor format change, and that would give us a 44 bit total
file size without much if any sweat. 

In terms of total addressable file size (assuming a sparse file), the
limits are caused by the current limits of using a 4k blocks size and
the number of blocks which can be addressed by using the triple
indirection block.  This turns into (10 + 10 + 10 + 12) = 42 bits,
assuming a 4k block size.  (It's a little bit more than that, since
you can also store 2^22 bytes using the double indirection block,
et. al., but for order of magnitude calculations it's basically 42
bits).
>From a total filesystem standpoint, we're using 32 bit block pointers,
which assuming the use of 4k (12 bits) block sizes, gives us a 44 bit
limitation for the total filesystem.

So the limitations of ext2 are somewhere between 41 and 44 bits,
depending on how you count things.  Given that 41 bits is 2 Terabytes,
and 44 bits is 16 Terabytes, it's not really a practical limitation at
this point.  

It's not clear, by the way, how often you really *want* to create
filesystems quite this huge.  Even on an EMC Symmetrix, for example,
which can support pedabytes of information today (if you don't mind
the multi-million dollar price tag), the typical usage pattern is to
create thousands and thousands of smaller filesystems, just simple for
easier manageability by the system administrators.  Hence when we
talked to EMC about what they needed most from Linux to better support
EMC boxes, the answer was not filesystems that could support
gargantuan disk sizes, but rather a SCSI subsystem that could support
thousands of LUN's....

						- Ted

Hi,

On Fri, Jul 06, 2001 at 11:44:51PM -0400, Iain Campbell
wrote:
> 
> I am doing some comparisons of different fs's and need to know where
ext3
> stands in terms of 64 bit addressing. XFS and JFS claim to be full 64 bit 
> systems (though are there not still kernel limitations that would, for the 
> time being, reduce the ability to use a full 64 bits?), what about ext3??
The same as ext2.  Basically ext2/3 support the full device size which
the kernel itself can support, which is 2^32 * 512 bytes or 2TB.  ext2
can go a little beyond this by using blocksizes larger than 512 bytes
(up to 4k at the moment, or 16TB.)

That 2TB limit is a hard limit for all filesystems, so ext2/3 is no more
limited than any other filesystem for now.

Things will change somewhat in the future.  Already, people are
working on removing that 2TB block device limit, and at that point
we'll need to add a mechanism to allow ext3 to address space beyond
16TB (which we can do in a fully forwards-compatible way due to the
ext2 superblock "feature" bits.)  XFS, on the other hand, is already
64-bit clean so won't need any new on-disk structures to go beyond 2TB
once the kernel is ready for that.  I don't know about JFS.

Cheers,
 Stephen