lkml.org 
[lkml]   [2009]   [Feb]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: ext3: call blkdev_issue_flush on fsync
From
Date
On Wed, 2009-01-28 at 10:55 +0100, Jan Kara wrote: 
> On Wed 28-01-09 18:45:13, Fernando Luis Vázquez Cao wrote:
> > 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.
> Yes, I came to this conclusion as well when we were discussing how to fix
> other filesystems which fail to issue flush in fsync
> (https://kerneltrap.org/mailarchive/linux-ext4/2009/1/22/4788004/thread).
> I think it makes sence there's a general mount option recognized at VFS
> level which would set superblock flag whether flush on fsync is needed or
> not. Then you could use this flag in your patch.
> So if you have time, please write a (separate) patch introducing this
> generic option. Thanks.

Sorry it's taken me so long to get back at you, but I got distracted
with some high priority stuff. I finally got around writing the patches
for both the kernel and util-linux which I am sending as a reply to this
email. Can you tell me if this is what you had in mind?

Regards,

Fernando

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2009-02-12 11:35    [W:0.194 / U:0.912 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site