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SubjectHow to handle >16TB devices on 32 bit hosts ??

Hi,
It has recently come to by attention that Linux on a 32 bit host does
not handle devices beyond 16TB particularly well.

In particular, any access that goes through the page cache for the
block device is limited to a pgoff_t number of pages.
As pgoff_t is "unsigned long" and hence 32bit, and as page size is
4096, this comes to 16TB total.

A filesystem created on a 17TB device should be able to access and
cache file data perfectly providing CONFIG_LBDAF is set.
However if the filesystem caches metadata using the block device,
then metadata beyond 16TB will be a problem.

Access to the block device (/dev/whatever) via open/read/write will
also cause problems beyond 16TB, though if O_DIRECT is used I think
it should work OK (it will probably try to flushed out completely
irrelevant parts of the page cache before allowing the IO, but that
is a benign error case I think).

With 2TB drives easily available, more people will probably try
building arrays this big and we cannot just assume they will only do
it on 64bit hosts.

So the question I wanted to ask really is: is there any point in
allowing >16TB arrays to be created on 32bit hosts, or should we just
disallow them? If we allow them, what steps should we take to make
the possible failure modes more obvious?

As I said, I think O_DIRECT largely works fine on these devices and
we could fix the few irregularities with little effort. So one step
might be to make mkfs/fsck utilities use O_DIRECT on >16TB devices on
32bit hosts.

Given that non-O_DIRECT can fail (e.g. in do_generic_file_read,
index = *ppos >> PAGE_CACHE_SHIFT
will lose data if *ppos is beyond 44 bits) we should probably fail
opens on devices larger than 16TB.... though just failing the open
doesn't help if the device can change size, as dm and md devices can.

I believe ext[234] uses the block device's page cache for metadata, so
they cannot safely be used with >16TB devices on 32bit. Is that
correct? Should they fail a mount attempt? Do they?

Are there any filesystems that do not use the block device cache and
so are not limited to 16TB on 32bit?

Even if no filesystem can use >16TB on 32bit, I suspect dm can
usefully use such a device for logical volume management, and as long
as each logical volume does not exceed 16TB, all should be happy. So
completely disallowing them might not be best.

I suppose we could add a CONFIG option to make pgoff_t be
"unsigned long long". Would the cost/benefit of that be acceptable?

Your thoughts are most welcome.

Thanks,
NeilBrown


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