On Tue, Nov 02, 2010 at 07:58:02AM +0000, Alex Bligh wrote: > On 2 Nov 2010, at 01:49, "Ted Ts'o" <tytso@xxxxxxx> wrote: > > But why not just use O_DIRECT? Do you really need to access the > > disk directly, as opposed to using O_DIRECT? > > > Because more than one machine will be accessing the data on the ext4 > volume (over iSCSI), though access to the large files is mediated by > locks higher up. To use O_DIRECT each accessing machine would need > to have the volume mounted, rather than merely receiving a list of > extents. Well, I would personally not be against an extension to fallocate() where if the caller of the syscall specifies a new flag, that might be named FALLOC_FL_EXPOSE_OLD_DATA, and if the caller either has root privs or (if capabilities are enabled) CAP_DAC_OVERRIDE && CAP_MAC_OVERRIDE, it would be able to allocate blocks whose extents would be marked as initialized without actually initializing the blocks first. I don't know whether it will get past the fs-devel bike shed painting crew, but I do have some cluster file system users who would like something similar. In their case they will be writing the files using Direct I/O, and the objects are all checksumed at the cluster file system level, and if the object has the wrong checksum, then the cluster file system will ask another server for the object. Since the cluster file system is considered trusted, and it verifies the expected object checksum before releasing the data, there is no security issue. You do realize, though, that it sounds like with your design you are replicating the servers, but not the disk devices --- so if your disk device explodes, you're Sadly Out of Luck. Sure you can use super-expensive storage arrays, but if you're writing your own cluster file system, why not create a design which uses commodity disks and worry about replicating data across servers at the cluster file system level? - Ted _______________________________________________ Ext3-users mailing list Ext3-users@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/ext3-users
