Re: New server: SSD/RAID recommendations?

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.

> *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