Re: Reliability with RAID 10 SSD and Streaming Replication

On 17.5.2013 03:34, Mark Kirkwood wrote:
> On 17/05/13 12:06, Tomas Vondra wrote:
>> Hi,
>>
>> On 16.5.2013 16:46, Cuong Hoang wrote:
>
>>> Pro for the master server. I'm aware of write cache issue on SSDs in
>>> case of power loss. However, our hosting provider doesn't offer any
>>> other choices of SSD drives with supercapacitor. To minimise risk, we
>>> will also set up another RAID 10 SAS in streaming replication mode. For
>>> our application, a few seconds of data loss is acceptable.
>>
>> Streaming replication allows zero data loss if used in synchronous mode.
>>
>
> I'm not sure synchronous replication is really an option here as it will
> slow the master down to spinning disk io speeds, unless the standby is
> configured with SSDs as well - which probably defeats the purpose of
> this setup.

The master waits for reception of the data, not writing them to the
disks. It will have to write them eventually (and that might cause
issues), but I'm not really sure it's that simple.

> On the other hand, if the system is so loaded that a pure SAS (spinning
> drive) solution can't keen up, then the standby lag may get to be way
> more than a few seconds...which means look out for huge data loss.

Don't forget the slave does not perform all the I/O (searching for the
row etc.). It's difficult to say how much this will save, though.

Tomas