[Lustre-devel] Erratum about indexes in robinhood DB

[Nathan Rutman]

We actually already did some of that for a one-off. We didn't push the changes upstream because
there were some ugly layering violations involved.  Vitaly, do you remember the details?


On Oct 11, 2011, at 6:04 AM, Eric Barton wrote:

> Thomas,
> 
> Interesting point about changelog entries requiring a 'stat'.
> 
> Nathan, what's your take on making changelogs tell you what has
> changed - even if only on "easy" changes?
> 
>         Cheers,
>                  Eric
> 
>> -----Original Message-----
>> From: LEIBOVICI Thomas [mailto:thomas.leibovici at cea.fr]
>> Sent: 06 October 2011 12:02 PM
>> To: Eric Barton
>> Cc: lustre-devel at lists.lustre.org
>> Subject: Re: Erratum about indexes in robinhood DB
>> 
>> Hello Eric,
>> 
>> With a fast enough feeder, the ingest rate robinhood can currently
>> sustain is between 50.000/sec and 100.000/sec
>> (depending on insert/update/remove ratio) with a basic MySQL DB stored
>> on a local disk.
>> This can certainly still be improved with MySQL tunings and/or better HW
>> and/or enterprise class DB,
>> but for now, we notice it is easily high enough for reading a MDT
>> changelog stream on a Petaflopic system.
>> 
>> This rate is actually lower when processing Lustre MDT changelogs (but I
>> have no measurement) because of "stat" operations to get file attributes
>> (unfortunately, changelogs do not give the new value of what has just
>> changed, e.g new uid for a chown operation, new size&mtime with a mtime
>> event...)
>> SOM will probably improve that point, but it could be a good idea to add
>> more info in changelogs.
>> 
>> Handling chglogs from multiple MDTs is indeed a very interesting point
>> to address.
>> The main issue is the database scaling in terms of operation rate,
>> volume and entry location.
>> A solution could be using an existing clustered DB engine (MySQL
>> cluster, NOSQL DBs...),
>> thus we are going to take a look at the different alternatives and see
>> if they could match the need.
>> For that, it would be interesting to know how records will be splitted
>> into the multiple changelog streams:
>> is a given fid always reported by the same stream? what about the parent
>> fid (like in create/unlink operations)?
>> If you have a document about DNE design, I think it would give a more
>> precise idea about
>> what event and fid is supposed to be reported by each MDT.
>> 
>> Thanks,
>> Thomas
>> 
>> Eric Barton wrote:
>>> Thomas,
>>> 
>>> Thanks a lot and I hope you don't mind me cc-ing lustre-devel as this
>>> seems to be of general interest.
>>> 
>>> Do you have a feel (or measurements :) for the rate at which a changelog
>>> can be ingested into robinhood?  And I'm wondering about DNE and multiple
>>> changelogs coming from multiple MDTs.  I'd be very interested to know if
>>> you've thought about this and have views on what the maximum ingest rate
>>> could be and whether there will be issues coordinating/merging events
>>> across multiple feeds.
>>> 
>>>         Cheers,
>>>                  Eric
>>> 
>>> Eric Barton
>>> CTO Whamcloud, Inc.
>>> 
>>> 
>>>> -----Original Message-----
>>>> From: LEIBOVICI Thomas [mailto:thomas.leibovici at cea.fr]
>>>> Sent: 29 September 2011 10:04 AM
>>>> To: Eric Barton
>>>> Subject: Erratum about indexes in robinhood DB
>>>> 
>>>> Hello Eric,
>>>> 
>>>> Re-thinking about your question on indexes in robinhood DB, my answer
>>>> was incomplete.
>>>> Actually, there are indexes on user/group/type/status, but there are not
>>>> on the main table:
>>>> 
>>>> 1) As I said you, on the main table (the one that list all FS entries),
>>>> there are as few indexes as possible (just fid as primary key, and
>>>> parent fid)
>>>> in order to preserve a good insert/update rate on this table whatever
>>>> the FS size (the deeper the DB index trees, the slower those requests).
>>>> 
>>>> 2) There is a secondary table where robinhood maintains aggregated
>>>> statitics like nbr entries, volume per user/group/type/(hsm)status and
>>>> which is updated on the fly.
>>>> This one as indexes on quite all its fields, which makes it possible to
>>>> get instantaneous stats per user, etc. without penalizing insert/update
>>>> rate on main table.
>>>> Indexes on this secondary table are less expensive, given that the set
>>>> of users is much more resticted that the nbr of entries.
>>>> 
>>>> This time you have a more complete answer.
>>>> 
>>>> Best regards
>>>> Thomas
>>>> 
>>> 
>>> 
> 
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