Re: Performance issue related to dropping (empty) tables

That’s 1.8m files, even with SSDs, it’ll take some time, when the WAL log is processed once in a while. Maybe you can drop the tables in smaller batches and drop them more often (i.e. don’t wait until you’ve gathered tables of 20 days to drop at once)? Jennie

On 23 Aug 2019, at 16:28, Gotwisner, Dave wrote:

Across all the tables, we have about 200 columns. This development machine is running on a machine with a raid of SSDs. From: users-list on behalf of Ying Zhang Sent: Friday, August 23, 2019 4:24 PM To: Communication channel for MonetDB users Subject: Re: Performance issue related to dropping (empty) tables

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Hai,

The long exec. time is probably caused by fdatasync.

How many columns do the tables contain?

What type of disk are you using?

Regards, Jennie

...

On 19 Aug 2019, at 13:04, Gotwisner, Dave wrote:

I am seeing a performance issue related to schema upgrades. This is from a C++ program that uses the MAPI library.

We try and keep 30 days worth of data, imported at a fairly high data rate with lots of columns. We use multiple threads to ingest data (bulk insert), and the number of threads is stored in a configuration table.

For 20 days worth of data, this works out to around 9000 tables (plus merge tables). We have roughly 6 sets of tables per hour, each of which has 4 tables and a merge table connecting them.

Because this is a schema change, we wrap the entire upgrade in a transaction.

For the purpose of the test, we had no data in the tables, but only had empty tables.

If we change our setting from 4 parallel tables to 2, we end up dropping 9000 tables. About half of the “drop table” commands take 0 seconds. The rest take anywhere between 4 and 12 seconds each.

Here is a plot of the time per delete, y access is seconds, x is the statement. The X access is NOT time based:

Any idea on how we can reduce the time? The total time to delete 9000 tables was 480 minutes.

Thanks,

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