Re: Ceph backfilling explained ( maybe )

[Loic Dachary <loic@dachary.org>]

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Hi !

On 05/25/2013 08:06 PM, Samuel Just wrote:
> Hi, thanks for taking the time to try to get all this documented!
> 
> Placement groups are assigned to a set of OSDs by crush.
> 
> (4.1, osdmap(e 1)) --CRUSH--> [3,1,2]
> 
> where the primary is 3.  When 3 dies, the osdmap is updated to reflect this
> and we get a new mapping for pg 4.1:
> 
> (4,1, osdmap(e 2)) --CRUSH--> [1,2,4]
> 
> Here, 1 and 2 already have up-to-date copies of 4.1.  osd 4, however, needs
> to be brought up to date.  During peering, osd 1 will learn that osd 4
> falls into
> 1 of 2 cases.
> 
> Case 1 is that osd 4 already had an old copy of pg 4.1 AND its pg log for pg
> 4.1 happens to overlap osd 1's pg log for pg 4.1.  In that case, by running
> through the log of operations, we can determine exactly which objects need
> to be copied over.  We usually refer to this as just "recovery" (or log based
> recovery).
> 
> In case 2, either osd 4's pg log does not overlap that of osd 1.  In this case,
> we cannot determine from the log which objects need to be copied over.
> To bring osd 4 up to date, we therefore need to backfill.
> 
> Backfill involves the primary and the backfill peer (there is only ever one in
> the acting set at a time, see PG::choose_acting) scanning over their pg stores
> and copying the objects which are different or missing from the primary to the
> backfill peer.  Because this may take a long time, we track the a last_backfill
> attribute for each local pg copy indicating how far the local copy has been
> backfilled.  In the case that the copy is complete, last_backfill is
> hobject_t::max().

Is it true that if two osd briefly disconnect while backfilling, they may be in the case 1 above (i.e. log based recovery ) and then backfilling again when done, starting from last_backfill and up ? 

> More exactly, a local pg copy is described by a few pieces of information:
> 1) the local pg log

pg_log_t https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1371
pg_log_entry_t https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1277

> 2) the local last_backfill

pg_info_t::last_backfill https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1102

> 3) the local last_complete

pg_info_t::last_complete https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1089

> 4) the local missing set

pg_missing_t https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1468

> The local pg store reflects all updates up to version last_complete on all

I assume you mean 'local pg log' instead of 'local pg log'. 

> hobject_ts hoid such that hoid < last_backfill AND hoid is not in the missing
> set.  Comparing the pg logs is used to fill in the missing set for OSDs which
> were only down for a brief period thus avoiding a costly backfill in many cases.

The pg logs are trimmed ( https://github.com/ceph/ceph/blob/master/src/osd/PG.cc#L216 ), this is why the pg logs of two OSDs that have been disconnected for too long are unlikely to overlap ? And therefore require a backfill because the two pg logs cannot be compared ?

> This is a bit of a rough brain dump and may be somewhat misleading/wrong.

It is very helpful as it is, thanks :-)

> I'll get it cleaned up and put it into
> doc/dev/osd_internals/pg_recovery.rst next
> week.
> 

That would be great. 

> Also, rados objects currently have three pieces:
> 1) data - read, write, writefull, etc.
> 2) xattrs
> 3) omap
> The omap is much like the xattrs except that it can generally store a much
> larger number of keys and support efficient scans.  It's used at the moment
> for a few things including rgw bucket indices.  The omap entries are copied
> over along with the rest of the object in recovery.  Behind the scenes, all
> omap entries for all objects stored on an OSD are stored prefixed in a single
> big leveldb instance.
> 
> omap operations probably shouldn't be supported on objects in an
> ErasureCodedPG :)

I thought omap / xattrs were mutually exclusive. I did not realize both could be used at the same time.

Cheers

> -Sam
> 
> On Sat, May 25, 2013 at 10:37 AM, Loic Dachary <loic@dachary.org> wrote:
>>
>>
>> On 05/25/2013 04:48 PM, Leen Besselink wrote:
>>> On Sat, May 25, 2013 at 04:27:16PM +0200, Loic Dachary wrote:
>>>>
>>>>
>>>> On 05/25/2013 02:33 PM, Leen Besselink wrote:
>>>> Hi Leen,
>>>>
>>>>> - a Cehp object can store keys/values, not just data
>>>>
>>>> I did not know that. Could you explain or give me the URL ?
>>>>
>>>
>>> Well, I got that impression from some of the earlier talks and from this blog post:
>>>
>>> http://ceph.com/community/my-first-impressions-of-ceph-as-a-summer-intern/
>>>
>>> But I haven't read it in while.
>>>
>>> But at this time I only see something like:
>>>
>>> http://ceph.com/docs/master/rados/api/librados/?highlight=rados_getxattr#rados_getxattr
>>>
>>> Which looks like it is storing it in filesystem attributes.
>>>
>>> So maybe an object can be a piece of data or a key/value store.
>>
>> Thanks for explaining: I did not know about the works of Eleanor Cawthon. I knew about the objects xattributes but I thought you meant that the data inside of the object could be structured as key/value pairs. My bad :-)
>>
>> Cheers
>>
>> --
>> Loïc Dachary, Artisan Logiciel Libre
>> All that is necessary for the triumph of evil is that good people do nothing.
>>

-- 
Loïc Dachary, Artisan Logiciel Libre
All that is necessary for the triumph of evil is that good people do nothing.


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