Re: [LSF/MM TOPIC] Fixing large block devices on 32 bit

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Fri, 2014-01-31 at 13:47 -0800, Dave Hansen wrote:
> On 01/31/2014 11:02 AM, James Bottomley wrote:
> >      3. Increase pgoff_t and the radix tree indexes to u64 for
> >         CONFIG_LBDAF.  This will blow out the size of struct page on 32
> >         bits by 4 bytes and may have other knock on effects, but at
> >         least it will be transparent.
> 
> I'm not sure how many acrobatics we want to go through for 32-bit, but...

That's partly the question: 32 bits was dying in the x86 space (at least
until quark), but it's still predominant in embedded.

> Between page->mapping and page->index, we have 64 bits of space, which
> *should* be plenty to uniquely identify a block.  We could easily add a
> second-level lookup somewhere so that we store some cookie for the
> address_space instead of a direct pointer.  How many devices would need,
> practically?  8 bits worth?

That might work.  8 bits would get us up to 4PB, which is looking a bit
high for single disk spinning rust.  However, how would the cookie work
efficiently? remember we'll be doing this lookup every time we pull a
page out of the page cache.  And the problem is that most of our lookups
will be on file inodes, which won't be > 16TB, so it's a lot of overhead
in the generic machinery for a problem that only occurs on buffer
related page cache lookups.

James


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