Re: Is file system replication sufficient to recovery?

Repeat after me: "RAID is not a backup". Write it down, chisel it into your
monitor, tattoo it on your arm. Yes, DRBD replicates data at the block
level but if you do a mass update and scramble your data in one place then
you still have two perfectly identical copies of bad data.

DRBD is pretty neat in that you create a block device under DRBD, that
device is replicated elsewhere, and then on top of that device you format a
filesystem or can use it as a raw device. As writes happen locally they
get shuffled to the other side.

--Paul D. Carlucci

On Thu, Dec 30, 2021, 2:31 PM Tom Korach <tom(at)safekeep(dot)com> wrote:

> > Maybe I'm missing something, but AFAIK plain old RAID will not protect
> > you against any scenario except failure of a single disk. It certainly
> > won't do anything to help you revert to a prior database state.
>
> The idea for RAID came from
> https://www.postgresql.org/docs/current/different-replication-solutions.html
> File System (Block Device) Replication
>
> A modified version of shared hardware functionality is file system
> replication, where all changes to a file system are mirrored to a file
> system residing on another computer. The only restriction is that the
> mirroring must be done in a way that ensures the standby server has a
> consistent copy of the file system — specifically, writes to the standby
> must be done in the same order as those on the primary. DRBD is a popular
> file system replication solution for Linux.
> DRBD seems to work similar to RAID but over network, but I might be wrong.
>
> According to that link (
> https://www.postgresql.org/docs/current/backup-file.html)
> > An alternative file-system backup approach is to make a “consistent
> snapshot” of the data directory, if the file system supports that
> functionality (and you are willing to trust that it is implemented
> correctly).
> > The typical procedure is to make a “frozen snapshot” of the volume
> containing the database
>
>
> So could the following backup architecture make sense?
> 1. Periodic snapshots using EBS mechanism (to get consistent snapshots).
> 2. Periodic pg_basebackup + WAL file archiving ( to allow reverting to a
> previous step if we e.g. mistakenly drop a table).
>
> Thanks,
> Tom
> On Thu, Dec 30, 2021 at 12:52 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
>> Tom Korach <tom(at)safekeep(dot)com> writes:
>> >> What do you mean exactly by "file-system replication"?
>>
>> > RAID1 setup (specifically, between two disks or EBS volumes [on AWS]),
>> > using LVM.
>>
>> Maybe I'm missing something, but AFAIK plain old RAID will not protect
>> you against any scenario except failure of a single disk. It certainly
>> won't do anything to help you revert to a prior database state.
>>
>> The docs page I pointed you to is part of a chapter that lays out all
>> the backup methods the PG community considers reliable. I strongly
>> suggest sticking to one of those and not trying to take shortcuts.
>> (The following chapter on high-availability setups is relevant
>> reading as well.)
>>
>> regards, tom lane
>>
>