On Tue, Feb 12, 2013 at 09:56:41PM +0100, Bernd Schubert wrote: > On 02/12/2013 09:28 PM, J. Bruce Fields wrote: > > 06effdbb49af5f6c "nfsd: vfs_llseek() with 32 or 64 bit offsets (hashes)" > > and previous patches solved problems with hash collisions in large > > directories by using 64- instead of 32- bit directory hashes in some > > cases. But it caused problems for users who assume directory offsets > > are "small". Two cases we've run across: > > > > - older NFS clients: 64-bit cookies cause applications on many > > older clients to fail. > > - gluster: gluster assumed that it could take the top bits of > > the offset for its own use. > > > > In both cases we could argue we're in the right: the nfs protocol > > defines cookies to be 64 bits, so clients should be prepared to handle > > them (remapping to smaller integers if necessary to placate applications > > using older system interfaces). And gluster was incorrect to assume > > that the "offset" was really an "offset" as opposed to just an opaque > > value. > > > > But in practice things that worked fine for a long time break on a > > kernel upgrade. > > > > So at a minimum I think we owe people a workaround, and turning off > > dir_index may not be practical for everyone. > > > > A "no_64bit_cookies" export option would provide a workaround for NFS > > servers with older NFS clients, but not for applications like gluster. > > > > For that reason I'd rather have a way to turn this off on a given ext4 > > filesystem. Is that practical? > > I think Ted needs to answer if he would accept another mount option. But > before we are going this way, what is gluster doing if there are hash > collions? They probably just haven't tested NFS with large enough directories. The birthday paradox says you'd need about 2^16 entries to have a 50-50 chance of hitting the problem. I don't know enough about ext4 directory performance. But unfortunately I suspect there's a range of directory sizes that are too small to have a significant chance of having directory collisions, but still large enough to need dir_index? --b. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
