On 11/24/2015 08:14 PM, Andres Freund wrote:
On 2015-11-24 13:09:58 -0600, Kevin Grittner wrote:
On Tue, Nov 24, 2015 at 12:48 PM, Marcin Mańk <marcin.mank@xxxxxxxxx> wrote:
if SSDs have 4kB/8kB sectors, and we'd make the Postgres page
size equal to the SSD page size, do we still need
full_page_writes?
If an OS write of the PostgreSQL page size has no chance of being
partially persisted (a/k/a torn), I don't think full page writes
are needed. That seems likely to be true if pg page size matches
SSD sector size.
At the very least it also needs to match the page size used by the
OS (4KB on x86).
Right. I find this possibility (when the OS and SSD page sizes match)
interesting, exactly because it might make the storage resilient to torn
pages.
But be generally wary of turning of fpw's if you use replication.
Not having them often turns a asynchronously batched write workload
into one containing a lot of synchronous, single threaded, reads.
Even with SSDs that can very quickly lead to not being able to keep
up with replay anymore.
I don't immediately see why that would happen? Can you elaborate?
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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