[Pvfs2-developers] CoalescingLowWatermark setting

[Sam Lang]

Pete Wyckoff wrote:
> slang at mcs.anl.gov wrote on Thu, 21 Sep 2006 11:20 -0500:
>> That said, the 
>> code you are referring to will behave in the way you described under 
>> 'low load' conditions.  If there are no other operations in the dbpf op 
>> queue marked TROVE_SYNC (or less than whatever LWM is set to) when that 
>> second check is made, we sync.
> 
> Thanks for the explanation.  I see what your intent is now with this
> low watermark, just to keep things a bit more in-sync when load is
> low.
> 
>> Just curious, you mentioned 5 calls to fdatasync() in a single create. 
>> That _should not_ happen, and is a bug if it does.  Its the db->sync 
>> call that we make 5 times (potentially, depending on parameters and 
>> load).  Are you seeing fdatasync() for metadata operations?  Also, have 
>> you see a big drop in metadata performance?
> 
> I meant single create from the client sysint point of view.  As you
> saw, that's three requests under the hood, with 1 + 2 + 2 = 5 total
> syncs across the three requests.
> 
> We did talk about transactions before.  Seems like the way to go if
> this becomes an issue for people in real life.  I'm just running
> benchmarks now.
> 
> Metadata performance has not dropped as far as I can tell.  It's
> just that we've been taking a closer look at MD operations and found
> big timing variations due to the presence of sync operations.  First
> cut was just to turn off disk effects, which led to my confusion
> about what the watermarks meant.  (They're marks on different water
> levels:  pending unprocessed sync-requiring ops, and total dirty
> unsynced ops present in database.)

Exactly.  They're admittedly not the best names, and some of the only 
options in server-config.c that I haven't documented yet.  If it makes 
sense to change the names to something else, I'll be all for it.  We 
could probably keep the old ones privately for backwards compatibility.

-sam

> 
> 		-- Pete
>