[ceph-users] Low traffic Ceph cluster with consumer SSD.

Vitaliy Filippov vitalif at yourcmc.ru
Sun Nov 25 06:17:47 PST 2018


>>> the real risk is the lack of power loss protection. Data can be
>>> corrupted on unflean shutdowns
>>
>> it's not! lack of "advanced power loss protection" only means lower iops
>> with fsync, but not the possibility of data corruption
>>
>> "advanced power loss protection" is basically the synonym for
>> "non-volatile cache"
>
> A few years ago - it was pretty common knowledge that if it didnt have
> capacitors - and thus Power-Loss-Protection, then an unexpected power-off
> could lead to data-loss situations. Perhapos I'm not updated with recent
> development. Is it a solved problem today in consumergrade SSD?
> .. any links to insight/testing/etc would be welcome.
>
> https://arstechnica.com/civis/viewtopic.php?f=11&t=1383499
> - does at least not support the viewpoint.

All disks (HDDs and SSDs) have cache and may lose non-transactional writes  
that are in-flight. However, any adequate disk handles fsync's (i.e SATA  
FLUSH CACHE commands). So transactional writes should never be lost, and  
in Ceph ALL writes are transactional - Ceph issues fsync's all the time.  
Another example is DBMS-es - they also issue an fsync when you COMMIT.

-- 
With best regards,
   Vitaliy Filippov


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