On Wed, Jul 6, 2016 at 4:48 PM, Scott Marlowe <scott.marlowe@gmail.com> wrote:
> On Wed, Jul 6, 2016 at 12:13 PM, Merlin Moncure <mmoncure@gmail.com> wrote:
>> Disabling write back cache for write heavy database loads will will
>> destroy it in short order due to write amplication and will generally
>> cause it to underperform hard drives in my experience.
>
> Interesting. We found our best performance with a RAID-5 of 10 800GB
> SSDs (Intel 3500/3700 series) that we got MUCH faster performance with
> all write caching turned off on our LSI MEgaRAID controllers. We went
> from 3 to 4ktps to 15 to 18ktps. And after a year of hard use we still
> show ~90% life left (these machines handle thousands of writes per
> second in real use) It could be that the caching was getting in the
> way of RAID calcs or some other issue. With RAID-1 I have no clue what
> the performance will be with write cache on or off.
Right -- by that I meant disabling the write back cache on the drive
itself, so that all writes are immediately flushed. Disabling write
back on the raid controller should be the right choice; each of these
drives essentially is a 'caching raid controller' for all intents and
purposes. Hardware raid controllers are engineered around performance
and reliability assumptions that are no longer correct in an SSD
world. Personally I would have plugged the drives directly to the
motherboard (assuming it's a got enough lanes) and mounted the raid
against mdadm and compared.
merlin
