On 20/05/25 12:58, Achilleas Mantzios wrote:
Στις 20/5/25 12:17, ο/η Moreno Andreo έγραψε:
On 19/05/25 20:49, Achilleas Mantzios wrote:
On 19/5/25 17:38, Moreno Andreo wrote:
On 19/05/25 14:41, Achilleas Mantzios wrote:
On 5/19/25 09:14, Moreno Andreo wrote:
On 16/05/25 21:33, Achilleas Mantzios wrote:
On 16/5/25 18:45, Moreno Andreo wrote:
Hi,
we are moving our old binary data approach, moving them
from bytea fields in a table to external storage (making
database smaller and related operations faster and smarter).
In short, we have a job that runs in background and copies data
from the table to an external file and then sets the bytea
field to NULL.
(UPDATE tbl SET blob = NULL, ref = 'path/to/file' WHERE id =
<uuid>)
This results, at the end of the operations, to a table that's
less than one tenth in size.
We have a multi-tenant architecture (100s of schemas with
identical architecture, all inheriting from public) and we are
performing the task on one table per schema.
So? toasted data are kept on separate TOAST tables, unless those
bytea cols are selected, you won't even touch them. I cannot
understand what you are trying to achieve here.
Years ago, when I made the mistake to go for a coffee and let my
developers "improvise" , the result was a design similar to what
you are trying to achieve. Years after, I am seriously
considering moving those data back to PostgreSQL.
The "related operations" I was talking about are backups and
database maintenance when needed, cluster/replica management,
etc. With a smaller database size they would be easier in timing
and effort, right?
Ok, but you'll lose replica functionality for those blobs, which
means you don't care about them, correct me if I am wrong.
I'm not saying I don't care about them, the opposite, they are
protected with Object Versioning and soft deletion, this should
assure a good protection against e.g. ransomware, if someone
manages to get in there (and if this happens, we'll have bigger
troubles than this).
PostgreSQL has become very popular because of ppl who care about
their data.
Yeah, it's always been famous for its robustness, and that's why I
chose PostgreSQL more than 10 years ago, and, in spite of how a
"normal" user treats his PC, we never had corruption (only where
FS/disk were failing, but that's not PG fault)
We are mostly talking about costs, here. To give things their
names, I'm moving bytea contents (85% of total data) to files
into Google Cloud Storage buckets, that has a fraction of the
cost of the disks holding my database (on GCE, to be clear ).
May I ask the size of the bytea data (uncompressed) ?.
single records vary from 150k to 80 MB, the grand total is more
than 8,5 TB in a circa 10 TB data footprint
This data is not accessed frequently (just by the owner when he
needs to do it), so no need to keep it on expensive hardware.
I've already read in these years that keeping many big bytea
fields in databases is not recommended, but might have
misunderstood this.
Ok, I assume those are unimportant data, but let me ask, what is
the longevity or expected legitimacy of those ? I haven't worked
with those just reading :
https://cloud.google.com/storage/pricing?_gl=1*1b25r8o*_up*MQ..&gclid=CjwKCAjwravBBhBjEiwAIr30VKfaOJytxmk7J29vjG4rBBkk2EUimPU5zPibST73nm3XRL2h0O9SxRoCaogQAvD_BwE&gclsrc=aw.ds#storage-pricing
would you choose e.g. "*Anywhere Cache storage" ?
*
Absolutely not, this is *not* unimportant data, and we are using
Standard Storage, for 0,02$/GB/month + operations, that compared to
a 0.17$/GB/month of an SSD or even more for the Hyperdisks we are
using, is a good price drop.
How about hosting your data in your own storage and spend 0$/GB/month ?
If we could host on our own hardware I'd not be here talking. Maybe
we would have a 10-node full-mesh multimaster architecture with
barman backup on 2 separate SANs.
But we are a small company that has to balance performance,
consistency, security and, last but not latter, costs. And margins
are tightening.
**
Another way would have been to move these tables to a different
tablespace, in cheaper storage, but it still would have been 3
times the buckets cost.
can you actually mount those Cloud Storage Buckets under a
supported FS in linux and just move them to tablespaces backed by
this storage ?
Never tried, I mounted this via FUSE and had some simple operations
in the past, but not sure it can handle database operations in
terms of I/O bandwidth
Why are you considering to get data back to database tables?
Because now if we need to migrate from cloud to on-premise, or
just upgrade or move the specific server which holds those data I
will have an extra headache. Also this is a single point of
failure, or best case a cause for fragmented technology introduced
just for the sake of keeping things out of the DB.
This is managed as an hierarchical disk structure, so the calling
server may be literally everywhere, it just needs an account (or a
service account) to get in there ,
and you are locked in a proprietary solution. and at their mercy of
any future increases in cost.
Since we cannot host on our hardware, the only thing is to keep an
eye on costs and migrate (yeah, more work) when it's becoming
expensive. Every solution is proprietary, if you want to run on
cloud. Even the VMs where PostgreSQL is running.
The problem is: this is generating BIG table bloat, as you may
imagine.
Running a VACUUM FULL on an ex-22GB table on a standalone test
server is almost immediate.
If I had only one server, I'll process a table a time, with a
nightly script, and issue a VACUUM FULL to tables that have
already been processed.
But I'm in a logical replication architecture (we are using a
multimaster system called pgEdge, but I don't think it will
make big difference, since it's based on logical replication),
and I'm building a test cluster.
So you use PgEdge , but you wanna lose all the benefits of
multi-master , since your binary data won't be replicated ...
I don't think I need it to be replicated, since this data cannot
be "edited", so either it's there or it's been deleted. Buckets
have protections for data deletions or events like ransomware
attacks and such.
Also multi-master was an absolute requirement one year ago
because of a project we were building, but it has been abandoned
and now a simple logical replication would be enough, but let's
do one thing a time.
Multi-master is cool, you can configure your pooler / clients to
take advantage of this for full load balanced architecture, but if
not a strict requirement , you can live without it, as so many of
us, and employ other means of load balancing the reads.
That's what we are doing, it's a really cool feature, but I
experienced (maybe because it uses old pglogical extension) that
the replication is a bit fragile, especially when dealing with
those bytea fields (when I ingest big loads, say 25-30 GB or more),
it happened to break replication, and recreating a replica from
scratch with "normal size" tables is not a big deal, since it can
be achieved automatically, because they normally fit in shared
memory and can be transferred by the replicator, but you can
imagine what would be the effort and the downtime necessary to
create a base backup, transfer it to the replica, build the DB and
restart a 10-TB database (ATM we are running with a 2-node cluster).
Break this in batches, use modern techniques for robust data
loading, in smaller transactions, if you have to.
Normally it's run via COPY commands, I can throttle COPY or break it
in batches. At the moment, while the schema is offline, we disconnect
replication from the bytea tables, feed them, wait for checkpoints to
return normal and then resume replication between tables before
putting schema online. This is safe, even if far from being
optimized. It's a migration tool, it won't be used forever, just to
move customers from their current architecture to new cloud one.
I've been instructed to issue VACUUM FULL on both nodes,
nightly, but before proceeding I read on docs that VACUUM FULL
can disrupt logical replication, so I'm a bit concerned on how
to proceed. Rows are cleared one a time (one transaction, one
row, to keep errors to the record that issued them)
Mind if you shared the specific doc ?
Obviously I can't find it from a quick search, I'll search deeper,
I don't think it went off a dream :-).
PgEdge is based on the old pg_logical, the old 2ndQuadrant
extension, not the native logical replication we have since
pgsql 10. But I might be mistaken.
Don't know about this, it keeps running on latest pg versions (we
are about to upgrade to 17.4, if I'm not wrong), but I'll ask
I read about extensions like pg_squeeze, but I wonder if they
are still not dangerous for replication.
What's pgEdge take on that, I mean the bytea thing you are
trying to achieve here.
They are positive, it's they that suggested to do VACUUM FULL on
both nodes... I'm quite new to replication, so I'm searching some
advise here.
As I told you, pgEdge logical replication (old 2ndquadrant BDR) !=
native logical replication. You may look here :
https://github.com/pgEdge/spock
If multi-master is not a must you could convert to vanilla
postgresql and focus on standard physical and logical replication.
No, multimaster is cool, but as I said, the project has been
discontinued and it's not a must anymore. This is the first step,
actually. We are planning to return to plain PostgreSQL, or
CloudSQL for PostgreSQL, using logical replication (that seems the
most reliable of the two). We created a test case for both the
options, and they seem to be OK for now, even if I have still to do
adequate stress tests. And when I'll do the migration, I'd like to
be migrating plain data only and leave blobs where they are.
as you wish. But this design has inherent data infra fragmentation
as you understand.
Personally I like to let the DB take care of the data, and I take
care of the DB, not a plethora of extra systems that we need to keep
connected and consistent.
We followed this idea when the application (old version) was on
customer premises, so backups and operations were simple and getting
in trouble (e.g. customer deleting a directory from their PC) has
happened a very few times, just when they launched disk cleanup on
windows :-)
Now we host a full cloud solution, so we got rid of many potential
problems generated by the end user, but bumped into other, as you
certainly imagine. We have to keep it consistent, fast, reliable,
keeping an eye on costs.
You are right, but the more I was working with this solution, the
more I'm having the impression of dealing with something heavy, hard
to mantain because of these rarely-accessed files that sum up most of
my data. Maybe it's just my impression, maybe I need some expertise
in an area that's still quite new for me.
At the moment that seems a good compromise between stability and
costs. Maybe in one year I'll be in your position (considering
getting everything back), but for now we are thinking forward in that
way.
Makes perfect sense.
This been said, the original question :-)
Would be VACUUM FULL a risky operation? Has it to be done on all
nodes, obviously in a low-traffic and low-access timing (night)?
VACUUM affects the physical blocks. In a physical streaming
replication scenario that might (or not) potentiallyt affect read-only
queries on the hot standby (depending on usage and settings). Normally
I cannot see how a VACUUM (plain or FULL) would interact with logical
replication in any way. But again, since you run PgEdge specific, you
have to ask them.
Thanks. This makes me think I misread or misinterpreted something. They
already suggested me that to use VACUUM FULL on both nodes, but that
"thing" I read (or I'm convinced to have) made me think twice before
crashing everything. Two experts' according words is quite enough for me.
I will start this evening and see what happens.
Thanks for the help and the very interesting discussion.
Thanks for your help.
Moreno.-
