Re: 2.5TB Migration from SATA to SSD disks - PostgreSQL 9.2

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On 9/7/16 6:10 PM, David Gibbons wrote:
> That is NOT safe. The problem is it allows rsync to use mtime alone
> to decide that a file is in sync, and that will fail if Postgres
> writes to a file in the same second that the first rsync reads from
> it (assuming Postgres writes after rsync reads). You need to add the
> --checksum flag to rsync (which means it will still have to read
> everything that's in /var/lib/pgsql).
>
>
> The checksum flag as you mention is not performant,

Definitely not. :/

> If this is a concern, you're much better using the *--modify-window *flag:
> When comparing two timestamps, rsync treats the timestamps as being
> equal if they differ by no more than the modify-window value. This is
> normally 0 (for an exact match), but you may find it useful to set this
> to a larger value in some situations.
>
> Hence, rsync -va --modify-window=1 would remove your concern about a
> same second race condition without forcing the sync to read through all
> the files.

Very interesting and useful!
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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