Re: [Documentation] Hardware recommandation : RAM and PGLog

[David Casier AEVOO <david.casier@aevoo.fr>]

OK,
I just understand the need of transactions for the trim takes place 
after changing settings.
What is the risk to have too low a value for the parameter 
osd_min_pg_log_entries (not osd_max_pg_log_entries in degraded 
environment) ?

David.

On 07/20/2015 03:13 PM, Sage Weil wrote:
> On Sun, 19 Jul 2015, David Casier AEVOO wrote:
>> Hi,
>> I have a question about PGLog and RAM consumption.
>>
>> In the documentation, we read "OSDs do not require as much RAM for regular
>> operations (e.g., 500MB of RAM per daemon instance); however, during recovery
>> they need significantly more RAM (e.g., ~1GB per 1TB of storage per daemon)"
>>
>> But in fact, all pg log are read in the start of ceph-osd daemon and put in
>> RAM ( pg->read_state(store, bl); )
>>
>> Is this normal behavior or I have a defect in my environment?
> There are two tunables that control how many pg log entries we keep
> around.  When teh PG is healthy, we keep ~1000, and when the PG is
> degraded, we keep more, to expand the time window over which a recovering
> OSD will be able to do regular log-based recovery instead of a more
> expensive backfill.  This is one source of additional memory.
>
> Others are the missing sets (lists of missing/degraded objects) and
> messages/data/state associated with objcts that are being
> recovered/copied.
>
> Note that the numbers in teh documentation are pretty rough rules of
> thumb.  At some point it would be great to build a model for how much RAM
> the osd consumes as a function of the various configurables (pg log size,
> pg count, avg object size, etc.).
>
> sage
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