On Tue, Jan 09, 2018 at 02:05:31PM -0500, Jeff Layton wrote: > > Ahh, many thanks... > > My main question was: why split the refcount across fields like this? If > it's necessary now for backward compatibility then so be it, but it's > weird and not 100% clear why it's being done that way. The main reason is that the only inode that will need it is the hidden extended attribute inode (and then only for Samba servers that are supporting enterprise domain CIFS servers where there are more than 64k files using the same windows ACL). So we didn't want to use extra bytes in the inode, since it's only going to be used in a very tiny fraction of servers. For the right workload, though, this should allow ext4 to have significantly better performance, since if you are serving a large directory where all of the files have their own ACL or Windows security ID xattrs, without shared extended attributes, when you open a Files explorer on that directory, for each file the file system will be forced to do lots of random 4k reads to fetch the xattrs. - Ted
