On Tue, Jul 7, 2015 at 10:59 AM, Heikki Linnakangas <hlinnaka@xxxxxx> 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.
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Mike Nolan
