Messages in this thread | | | Subject | Re: ext3: call blkdev_issue_flush on fsync | From | Fernando Luis Vázquez Cao <> | Date | Wed, 28 Jan 2009 18:45:13 +0900 |
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On Mon, 2009-01-19 at 13:03 +0100, Jan Kara wrote: > On Sat 17-01-09 19:00:49, Fernando Luis Vázquez Cao wrote: > > On Sat, 2009-01-17 at 18:47 +0900, Fernando Luis Vázquez Cao wrote: > > > On Fri, 2009-01-16 at 17:30 +0100, Jan Kara wrote: > > > > On Fri 16-01-09 22:55:01, Fernando Luis Vázquez Cao wrote: > > > > > To ensure that bits are truly on-disk after an fsync or fdatasync, we > > > > > should force a disk flush explicitly when there is dirty data/metadata > > > > > and the journal didn't emit a write barrier (either because metadata is > > > > > not being synched or barriers are disabled). > > > > > > > > > > Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp> > > > > > --- > > > > Only two minor nits: > > > > > > > > > --- linux-2.6.29-rc1-orig/fs/ext3/fsync.c 2008-12-25 08:26:37.000000000 +0900 > > > > > +++ linux-2.6.29-rc1/fs/ext3/fsync.c 2009-01-16 22:18:53.000000000 +0900 > > > > > @@ -27,6 +27,7 @@ > > > > > #include <linux/sched.h> > > > > > #include <linux/writeback.h> > > > > > #include <linux/jbd.h> > > > > > +#include <linux/blkdev.h> > > > > > #include <linux/ext3_fs.h> > > > > > #include <linux/ext3_jbd.h> > > > > > > > > > > @@ -45,6 +46,8 @@ > > > > > int ext3_sync_file(struct file * file, struct dentry *dentry, int datasync) > > > > > { > > > > > struct inode *inode = dentry->d_inode; > > > > > + journal_t *journal = EXT3_SB(inode->i_sb)->s_journal; > > > > > + unsigned long i_state = inode->i_state; > > > > > int ret = 0; > > > > > > > > > > J_ASSERT(ext3_journal_current_handle() == NULL); > > > > > @@ -69,23 +72,33 @@ int ext3_sync_file(struct file * file, s > > > > > */ > > > > > if (ext3_should_journal_data(inode)) { > > > > > ret = ext3_force_commit(inode->i_sb); > > > > > + if (!(journal->j_flags & JFS_BARRIER)) > > > > > + goto no_journal_barrier; > > > > > goto out; > > > > > } > > > > > > > > > > - if (datasync && !(inode->i_state & I_DIRTY_DATASYNC)) > > > > > - goto out; > > > > > + if (datasync && !(i_state & I_DIRTY_DATASYNC)) > > > > > + goto flush_blkdev; > > > > > > > > > > /* > > > > > * The VFS has written the file data. If the inode is unaltered > > > > > * then we need not start a commit. > > > > > */ > > > > > - if (inode->i_state & (I_DIRTY_SYNC|I_DIRTY_DATASYNC)) { > > > > > + if (i_state & (I_DIRTY_SYNC|I_DIRTY_DATASYNC)) { > > > > > struct writeback_control wbc = { > > > > > .sync_mode = WB_SYNC_ALL, > > > > > .nr_to_write = 0, /* sys_fsync did this */ > > > > > }; > > > > > ret = sync_inode(inode, &wbc); > > > > > + if (journal && !(journal->j_flags & JFS_BARRIER)) > > > > > + goto no_journal_barrier; > > > > I cannot imagine "journal" will be NULL here. > > > > > > I'll try to check whether that is always so just in case. > > > > > > > And we can also optimize here a bit and do "goto out" because here > > > > we know the barrier has been issued. > > > > > > Yep, I was considering the same optimization. By the way, I was > > > wondering if we should honor ext3 and ext4's "barrier" mount option for > > > sys_fsync()/sys_fdatasync() and do not force a flush when "barrier=1". > > > > The last phrase should read " do not force a flush when "barrier=0" ". > I was hesitating about this a bit. But I don't think so. The reason is > that POSIX (or any other reasonable specification) mandates that fsync() > should return only after the data is safely on storage. So if we don't > flush blockdevice caches, we effectively violate POSIX and we should never > do that. With barriers the matter is a bit different - that is just a > filesystem specific thing, no standard guarantees anything.
Hi Jan,
Sorry it's taken me so long to get back to you.
Thinking a lit bit more about this issue, it occurred to me that adding a new mount option à la existing "barrier" is likely to be preferable. As an example where such an option could make sense, let's consider a system with battery-backup cache devices. Since the battery-backup guarantees the data still not committed to the platter will not vanish in the event of a power down, it should be possible to obtain a performance gain by optimizing out the device flush on every fsync()/fdatasync() call.
If there is consensus on the propriety of this approach, I will send updated patches.
Regards,
Fernando
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