Lets say you have a table in a database that you are using to keep track of a counter value. So something like:
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If you have lots of connections updating values doing the query:
test set count = count + 1 where id = $1; then on postgresql you should have
very decent performance as long as the id values do not overlap too much. Even
though postgresql needs to fsync the WAL to disk for each commit it is able to
amortise the cost of this over many commits if the commits start to queue up
because fsyncing is slow. For example the WAL fsyncs might be something like:
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However, if the id values overlap and in the worst case if they are all the same then not only do you have a problem with lock contention but you also have a problem with serializing all of the fsyncs. Postgresql and presumably any sane RDBMS will hold a write lock on the updated records until the transaction is durable. So you end up getting all the WAL fsyncs done in a completely serial order.
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Before the land of SSDs this would be absolutely horrible. If you are paying 6ms for a fsync then it completely destroys your throughput (166 fsync/s). But now with SSDs (or previously with battery backed caches) the fsync cost is much lower so this is less of an issue. For example with Amazon EBS I see fsync cost of around 0.5ms (2000 fsync/s) and i3 NVME performance of ~0.05ms (20000 fsync/s).
Is it possible for an RDBMs to fix this fsync problem? So when you think about it an RDBMs could drop all the locks a transaction has once it has decided that nothing except for the WAL fsync would prevent it from committing. This would kind of work because another transaction that would modify the same row would be dependent on the previous WAL segments committing before it could commit. However, this opens up a big consistency hole in the way clients interact with the database. For example you could see this happening:
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Here we see a case where the second transaction observes data in the database that was not durable. It might think that because the record is already in the database it can do something else with an external system and then we end up having a problem. This particular case is also weird because the transaction gets in a state where it can’t be committed. If you successfully commit a transaction that has touched non-durable records then all the reads are safe because the records would now be durable after the commit. But a transaction with an error is not committable so you would also need to add a weird hook where a rollback (or implicit rollback) might have to wait for other transactions to fsync before returning to the client.
Also, transactions that did not modify data would normally have a no-op commit but if they were shown non-durable records they would need to potentially wait to commit.
Basically, it could kind of work as long as all transactions waited for a successful commit/rollback before acting on any data they read from the DB but this does not seem realistic.
If you look at VoltDB it looks like they only let you do transactions inside stored procedures. I’ve also seen a comment along the lines that they do batch commits. Considering, they are single threaded presumably they handle a bunch of transactions on the thread and add them to a queue that is fsynced in batches then the results from the stored procedures are sent back to the client. This presumably removes any of the consistency problems you have from a system that has external transactions where non-durable reads can escape.
If you want to play around with postgresql to see what effects wal fsync delay has I have a LD_PRELOAD library that will add 10s to every fsync. wal_delayer