Re: ext3: kjournald and spun-down disks

[Andrew Morton <akpm@zip.com.au>]

Daniel Kobras wrote:
> 
> On Fri, Nov 23, 2001 at 05:25:46PM -0800, Andrew Morton wrote:
> > Also, if we had appropriate hooks into the request layer, we could detect
> > when the disk was being spun up for a read, and opporunistically flush
> > out any pending writes.
> 
> Actually you can't. SCSI spinup code isn't very useful anyway, and IDE disks
> mostly handle spinup themselves. The kernel has too issue a reset to get a
> disk back alive from sleep mode, but revival from standby doesn't involve
> the kernel at all. When using the disk's internal timer, it isn't involved in
> spindown either. Teaching the request layer about disk state might therefore
> turn out to become rather messy, I suspect.

Much simpler approach:

	if (we're about to read from the disk) {
		if (we have dirty data which is > 10 seconds old) {
			write_it_now();
		}
	}


> > Tell me if this is joyful:
> [...]
> > -     transaction->t_expires = jiffies + journal->j_commit_interval;
> > +     transaction->t_expires = jiffies + dirty_buffer_flush_interval();
> 
> This change doesn't take care of kupdated's most interesting feature, i.e.
> that you can entirely stop it (with a flush interval of zero and/or a
> SIGSTOP).

yup.

> Now, if kjournald honoured SIGSTOP/SIGCONT, I could teach noflushd
> to handle the spindown issue in userland. Uh, at least for one small detail:
> Is there a way to tell which kjournald process is associated to which
> partition? A fake cmdline, or an fd to the partition's device node that
> shows up in /proc/<pid>/fd would indeed be quite helpful.

Andreas has a patch which puts the device major/minor into kjournald's
process name.

Simply setting the journal timer to infinity happens to work out OK.
Commits are triggered by kupdate.

This is because kupdate's superblock writeout runs a commit.  Because
ext3 is unable to distinguish it from a sys_sync().  Sigh.

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