Ext4 and large (>8TB) files

[Andreas Dilger]

On 2010-03-26, at 19:32, Arun Nair wrote:
> Ok, so I guess ext4 with 64-bit, or another filesystem for us.  
> Thanks Andreas & Eric for all the help.

No, I don't think another filesystem will help, on a 32-bit host.  The  
limit that ext4 is reporting is the VM page cache limit for a single  
file, and has nothing to do with ext4 itself.

> On Fri, Mar 26, 2010 at 3:38 PM, Andreas Dilger <adilger at dilger.ca>  
> wrote:
> On 2010-03-26, at 16:05, Arun Nair wrote:
> So I'm guessing switching the system to 64-bit would fix this for  
> us. How about increasing the block size from the current 4k? Would  
> that be an option too?
>
> Not in the near future, unless you are running on PPC/ARM/SPARC that  
> can also handle large pages.
>
> On Fri, Mar 26, 2010 at 2:10 PM, Eric Sandeen <sandeen at redhat.com>  
> wrote:
> On 03/26/2010 03:50 PM, Arun Nair wrote:
> > Eric,
> >
> > Thanks for the quick reply... see my responses inline...
> >
> > On Fri, Mar 26, 2010 at 12:16 PM, Eric Sandeen <sandeen at redhat.com
> > <mailto:sandeen at redhat.com>> wrote:
> >
> >     On 03/26/2010 01:52 PM, Arun Nair wrote:
> >     > Hi -
> >     >
> >     > (I apologize for the ext4 question in an ext3 mailer, but I  
> couldn't
> >     > find a user list for ext4.)
> >
> >     linux-ext4 at vger.kernel.org <mailto:linux- 
> ext4 at vger.kernel.org> :)
> >      but that's ok.
> >
> >
> > Saw that but thought it was a dev-only list, sorry. Next time :)
>
> *shrug* I think user questions are welcome too.  At least I don't  
> mind.
>
> ...
>
> > dd fails as mentioned above. xfs_io errors too:
> > [root at camanoe]# xfs_io -F -f -c "pwrite 8T 1M" bigfile2
> > pwrite64: File too large
>
> Oh.  Well, then!  Must be something else.
>
> oh, ok:
>
> sbi->s_bitmap_maxbytes = ext4_max_bitmap_size();
>
> static loff_t ext4_max_size(int blkbits, int has_huge_files)
> {
>       loff_t res;
>       loff_t upper_limit = MAX_LFS_FILESIZE;
>
> <snip>
>
>       /* Sanity check against vm- & vfs- imposed limits */
>       if (res > upper_limit)
>               res = upper_limit;
>
>       return res;
> }
>
> and:
>
> /* Page cache limit. The filesystems should put that into their  
> s_maxbytes
>  limits, otherwise bad things can happen in VM. */
> #if BITS_PER_LONG==32
> #define MAX_LFS_FILESIZE (((u64)PAGE_CACHE_SIZE <<  
> (BITS_PER_LONG-1))-1)
>
> so it's only giving us 31 bits of pages, not 32.  This limits it to 8T
> on a 32-bit machine with 4k pages.
>
> I'm not honestly sure if there is anything in the vm that can't  
> actually
> cope with a 32-bit offset... but until proven otherwise, probably not
> going to change this without a lot of testing & inspection.
>
> -Eric
>
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>
>
> Cheers, Andreas
>
>
>
>
>
>


Cheers, Andreas