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

On Wed, Sep 7, 2016 at 4:49 PM, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com> wrote:

> 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!

Isn't this heading in the wrong direction? We need to be more precise
than 0 (since 0 is computed off of rounded/truncated time stamps), not less
precise than 0.

Cheers,

Jeff