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The problem for atime: We have 3 places for atime: inode->i_atime, znode->z_atime and SA. And its handling is a mess. A huge part of mess regarding atime comes from zfs_tstamp_update_setup, zfs_inode_update, and zfs_getattr, which behave inconsistently with those three values. zfs_tstamp_update_setup clears z_atime_dirty unconditionally as long as you don't pass ATTR_ATIME. Which means every write(2) operation which only updates ctime and mtime will cause atime changes to not be written to disk. Also zfs_inode_update from write(2) will replace inode->i_atime with what's inside SA(stale). But doesn't touch z_atime. So after read(2) and write(2). You'll have i_atime(stale), z_atime(new), SA(stale) and z_atime_dirty=0. Now, if you do stat(2), zfs_getattr will actually replace i_atime with what's inside, z_atime. So you will have now you'll have i_atime(new), z_atime(new), SA(stale) and z_atime_dirty=0. These will all gone after umount. And you'll leave with a stale atime. The problem for relatime: We do have a relatime config inside ZFS dataset, but how it should interact with the mount flag MS_RELATIME is not well defined. It seems it wanted relatime mount option to override the dataset config by showing it as temporary in `zfs get`. But at the same time, `zfs set relatime=on|off` would also seems to want to override the mount option. Not to mention that MS_RELATIME flag is actually never passed into ZFS, so it never really worked. How Linux handles atime: The Linux kernel actually handles atime completely in VFS, except for writing it to disk. So if we remove the atime handling in ZFS, things would just work, no matter it's strictatime, relatime, noatime, or even O_NOATIME. And whenever VFS updates the i_atime, it will notify the underlying filesystem via sb->dirty_inode(). And also there's one thing to note about atime flags like MS_RELATIME and other flags like MS_NODEV, etc. They are mount point flags rather than filesystem(sb) flags. Since native linux filesystem can be mounted at multiple places at the same time, they can all have different atime settings. So these flags are never passed down to filesystem drivers. What this patch tries to do: We remove znode->z_atime, since we won't gain anything from it. We remove most of the atime handling and leave it to VFS. The only thing we do with atime is to write it when dirty_inode() or setattr() is called. We also add file_accessed() in zpl_read() since it's not provided in vfs_read(). After this patch, only the MS_RELATIME flag will have effect. The setting in dataset won't do anything. We will make zfstuil to mount ZFS with MS_RELATIME set according to the setting in dataset in future patch. Signed-off-by: Chunwei Chen <david.chen@osnexus.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Issue #4482 |
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Native ZFS for Linux!
ZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the Illumos community.
ZFS on Linux, which is also known as ZoL, is currently feature complete. It includes fully functional and stable SPA, DMU, ZVOL, and ZPL layers.
Full documentation for installing ZoL on your favorite Linux distribution can be found at: http://zfsonlinux.org