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

2016-09-08 11:49 GMT+12:00 Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>:

> Please include the mailing list in replies...
>
> 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!
> <http://www.postgresql.org/mailpref/pgsql-general>
>

Cool! I'll use the rsync -va --modify-window=1 instead.

Thanks!
Patrick