Re: New server: SSD/RAID recommendations?

On Tue, Jul 7, 2015 at 10:59 AM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:

> On 07/07/2015 05:15 PM, Wes Vaske (wvaske) wrote:
>
>> The M500/M550/M600 are consumer class drives that don't have power
>> protection for all inflight data.* (like the Samsung 8x0 series and
>> the Intel 3x0 & 5x0 series).
>>
>> The M500DC has full power protection for inflight data and is an
>> enterprise-class drive (like the Samsung 845DC or Intel S3500 & S3700
>> series).
>>
>> So any drive without the capacitors to protect inflight data will
>> suffer from data loss if you're using disk write cache and you pull
>> the power.
>>
>
> Wow, I would be pretty angry if I installed a SSD in my desktop, and it
> loses a file that I saved just before pulling the power plug.
>

That can (and does) happen with spinning disks, too.

>
> *Big addendum: There are two issues on powerloss that will mess with
>> Postgres. Data Loss and Data Corruption. The micron consumer drives
>> will have power loss protection against Data Corruption and the
>> enterprise drive will have power loss protection against BOTH.
>>
>>
>> https://www.micron.com/~/media/documents/products/white-paper/wp_ssd_power_loss_protection.pdf
>>
>> The Data Corruption problem is only an issue in non-SLC NAND but
>> it's industry wide. And even though some drives will protect against
>> that, the protection of inflight data that's been fsync'd is more
>> important and should disqualify *any* consumer drives from *any*
>> company from consideration for use with Postgres.
>>
>
> So it lies about fsync()... The next question is, does it nevertheless
> enforce the correct ordering of persisting fsync'd data? If you write to
> file A and fsync it, then write to another file B and fsync it too, is it
> guaranteed that if B is persisted, A is as well? Because if it isn't, you
> can end up with filesystem (or database) corruption anyway.
>
> - Heikki
>
>
The sad fact is that MANY drives (ssd as well as spinning) lie about their
fsync status.
--
Mike Nolan