2011/5/10 Greg Smith <greg@xxxxxxxxxxxxxxx>: > On 05/09/2011 11:13 PM, Shaun Thomas wrote: >> >> Take a look at /proc/sys/vm/dirty_ratio and >> /proc/sys/vm/dirty_background_ratio if you have an older Linux system, or >> /proc/sys/vm/dirty_bytes, and /proc/sys/vm/dirty_background_bytes with a >> newer one. >> On older systems for instance, those are set to 40 and 20 respectively >> (recent kernels cut these in half). > > 1/4 actually; 10% and 5% starting in kernel 2.6.22. The main sources of > this on otherwise new servers I see are RedHat Linux RHEL5 systems running > 2.6.18. But as you say, even the lower defaults of the newer kernels can be > way too much on a system with lots of RAM. one can experiment writeback storm with this script from Chris Mason, under GPLv2: http://oss.oracle.com/~mason/fsync-tester.c You need to tweak it a bit, AFAIR, this #define SIZE (32768*32) must be reduced to be equal to 8kb blocks if you want similar to pg write pattern. The script does a big file, many small fsync, writing on both. Please, see http://www.spinics.net/lists/linux-ext4/msg24308.html It is used as a torture program by some linuxfs-hackers and may be useful for the OP on his large server to validate hardware and kernel. > > The main downside I've seen of addressing this by using a kernel with > dirty_bytes and dirty_background_bytes is that VACUUM can slow down > considerably. It really relies on the filesystem having a lot of write > cache to perform well. In many cases people are happy with VACUUM > throttling if it means nasty I/O spikes go away, but the trade-offs here are > still painful at times. > > -- > Greg Smith 2ndQuadrant US greg@xxxxxxxxxxxxxxx Baltimore, MD > PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us > "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books > > > -- > Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance > -- Cédric Villemain 2ndQuadrant http://2ndQuadrant.fr/ ; PostgreSQL : Expertise, Formation et Support -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
