Re: Sysfs update frequency

[Justin Maggard <jmaggard10@gmail.com>]

On Tue, Mar 16, 2010 at 2:52 PM, Neil Brown <neilb@suse.de> 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.
>
> 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 this a problem for you?
>

Thanks for the info.  No, it's not much of a problem, really.  Just
seemed strange that an array of 2TB disks could resync for an hour
with no update to sync_completed.  I thought I remembered older
kernels updating a lot more frequently, but I could be wrong about
that.  So I take it that point is where the resync would resume if the
system was rebooted?

-Justin
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