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@xxxxxxxxxxxx> wrote:
So could the following backup architecture make sense?> 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.htmlFile 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 database1. 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,TomOn Thu, Dec 30, 2021 at 12:52 PM Tom Lane <tgl@xxxxxxxxxxxxx> wrote:Tom Korach <tom@xxxxxxxxxxxx> 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
