Re: Sysfs update frequency

[Michael Evans <mjevans1983@gmail.com>]

On Tue, Mar 16, 2010 at 3:25 PM, Justin Maggard <jmaggard10@gmail.com> wrote:
> 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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Rather than a time basis would it be possible to have a sysfs
paramater which could be tuned via write?

Candidates for this would be something like:

sync_flushes_per_action (fractional unit, every 1/N of the device)

OR

sync_flush_stripes
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