The comment on HDDs is true and gave me another thought.
These new 'shingled' HDDs (the 8TB ones) rely on rewriting all the data on tracks that overlap your data, any time you
changethe data. Result: disks 8-20x slower during writes, after they fill up.
Do they have power loss protection for the data being rewritten during reshingling? You could have data commited at
positionX and you accidentally nuke data at position Y.
[I know that using a shingled disk sounds crazy (it sounds crazy to me) but you can bet there are people that just want
tomax out the disk bays in their server... ]
Graeme.
On 07 Jul 2015, at 19:28, Michael Nolan <htfoot@gmail.com> wrote:
>
>
> On Tue, Jul 7, 2015 at 10:59 AM, Heikki Linnakangas <hlinnaka@iki.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
thepower 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'ddata? If you write to file A and fsync it, then write to another file B and fsync it too, is it guaranteed that
ifB 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
>
