Details
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Type:
Bug
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Status: Closed
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Resolution: Fixed
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Affects Version/s: None
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Fix Version/s: None
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Component/s: None
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Labels:
Description
As we know, query_cache overhead for LOCK can only be avoided when start-up mysql with query_cache_type is 0. That's what Percona contributed: http://www.percona.com/doc/percona-server/5.5/performance/query_cache_enhance.html?id=percona-server:features:query_cache_enhance#disabling_the_cache_completely
That's a nice work. However, if we want the query cache back, the mysqld must be restarted with query_cache_type=1. That's the problem, and some customers faced the scenarios.
I looked into the code, and found it might be resolved without the limitation of restarted mysqld. If we call is_disabled with a traced query_cache_type and combined with current setting query_cache_type, it's Okay to judge whether we could get the same result as old is_disabled function in Query_cache class.
Detail info could refer to qc.patch, which is based on Percona-5.5.18
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Re: dynamic setting query_cache_type but keeping the benefit of keeping away of overhead LOCK if query cache type is 0
Unpatched Percona could not dynamic swtith from OFF to ON, but MariaDB would, that's what you mentioned, and MariaDB got the same result for :
./mtr -record --mysqld="-query_cache_type=ON" t/qc.test
./mtr -record --mysqld="-query_cache_type=OFF" t/qc.test
Both warnings would be shown in error log.
Re: dynamic setting query_cache_type but keeping the benefit of keeping away of overhead LOCK if query cache type is 0
I enabled logging of locking in Query cache and checked that the lock taken only when QC is ON, when QC is off MariaDB do not try to take lock.
Re: dynamic setting query_cache_type but keeping the benefit of keeping away of overhead LOCK if query cache type is 0
Yes, I walked through the code of QC in MariaDB-5.5.25, it has fixed this issue with a more smart idea. Nice work, Sanja.
Re: dynamic setting query_cache_type but keeping the benefit of keeping away of overhead LOCK if query cache type is 0
Tested with MariaDB 5.5 and works as expected.
I noticed that there is a tracking variable used: m_requests_in_progress which is to make sure that disabling/enabling is not done while queries are in-flight within the query cache critical section.
I see that the enabling the a disabled query cache can fail only when
a) query cache disabling is in progress or
b) global query cache is disabled but session query_cache_type is tried.
Launchpad bug id: 1021131
qc.patch
LPexportBug1021131_qc.patch