Question

MyISAM uses table level locking which means that SELECT:s are blocked while INSERT/UPDATE:s are running.

To mitigate the problem of blocked SELECT:s I’ve been recommended to configure MySQL with these parameters:

• low_priority_updates=1
• concurrent_insert=2

What are the drawbacks of using low_priority_updates=1 and concurrent_insert=2?

Answer 1

Here’s a great post from the MySQL Performance Blog which covers some of this

Lock priorities. By default MySQL treats updates as higher priority
operations. You can use SELECT
HIGH_PRIORITY or UPDATE LOW_PRIORITY
to adjust that or you can simply set
low_priority_updates option. Anyway
default behavior means any UPDATE
statement which is blocked by long
running select will also block further
selects from this table – they will
have to wait until UPDATE is executing
which is waiting on SELECT to
complete. This is often not accounted
for and people think – “OK. I write my
script so it does short updates so it
will not block anything” – it still
may cause total block if there are
long selects running.

Another post benchmarks concurrent_inserts and highlights possible downsides, though the post is 3 years old now.