Re: Sysfs update frequency

[Neil Brown <neilb@suse.de>]

On Sat, 20 Mar 2010 12:48:28 -0400
Bill Davidsen <davidsen@tmr.com> wrote:

> Neil Brown wrote:
> > On Tue, 16 Mar 2010 14:32:55 -0700
> > Justin Maggard <jmaggard10@gmail.com> wrote:
> >
> >   
> >> I've noticed on recent kernels that /sys/block/md?/md/sync_completed
> >> seems to rarely get updated.  What is the expected update interval?
> >> For me, it seems to only update about once every 6% or so during the
> >> resync.  Of course, /proc/mdstat has the actual current progress.
> >>     
> >
> > The expected update time is every 6% - actually 1/16 which is 6.25%.
> >
> > sync_completed includes a guarantee that all blocks before this point really
> > have been processed.  The number in /proc/mdstat is less precise.  The much
> > of the array has been resynced, but due to the possibility of out-of-order
> > completion of writes they may not be a contiguous series of blocks.
> >
> >   
> Couldn't you just track the outstanding writes by LBA (or similar) and 
> report that the completion is one less than the lowest write still 
> outstanding? Since you would only do it when the user requests it, I 
> don't think the overhead of a list scan or similar would be a show 
> stopper. Or is that approach too simplistic?

I'd have to create a data structure to which I add and remove these LBAs at a
significant rate.  It isn't really worth the effort.

> 
> > Providing the guarantee (which is needed for externally-managed metadata)
> > requires briefly stalling the resync, so I didn't want to do it more often.
> > I could possibly make it time-bases instead of size-based though.
> >   
> 
> Is perfect accuracy needed, just as long as you don't promise to have 
> synced more than you have? Are you using barriers to be sure the data is 
> all the way to the platter, or is your stall just "to the device" 
> anyway? Like any snapshot of a dynamic process, by the time you get the 
> information it's out of date in any case, so I think a "at least this 
> much has moved to the device" value would serve.
> 

The information may be used to update metadata, so it is critical that it
doesn't say more than is true.  It is safe for it to say less than is true.

A metadata update would always be preceded by a barrier so that the data on
the device is consistent.

"at least this much has moved" isn't much good if it only tells us how many
blocks, not which ones.
The value in sync_completed says "at least all the blocks up to this one have
been synced" which is exactly the information that I want.

NeilBrown