Re: [PERFORM] VX_CONCURRENT flag on vxfs( 5.1 or later) for performance for postgresql?
You should rather consider VxFS tuning - it has an auto-discovery for DIRECT I/O according the block size. Just change this setting to 8K or 16-32K depending on your workload - then all I/O operations with a bigger block size will be executed in DIRECT mode and bypass FS cache (which logical as usually it'll correspond to a full scan or a seq scan of some data), while I/O requests with smaller blocks will remain cached which is very useful as it'll mainly cache random I/O (mainly index access).. With such a tuning I've got over %35 performance improvement comparing to any other states (full DIRECT or fully cached). Rgds, -Dimitri Rgds, -Dimitri On 5/5/11, Robert Haas robertmh...@gmail.com wrote: On Sat, Apr 30, 2011 at 4:51 AM, Hsien-Wen Chu chu.hsien@gmail.com wrote: since the block size is 8k for the default, and it consisted with many tuple/line; as my understand, if any tuple/line is changed(maybe update, insert, delete). the block will be marked as dirty block. and then it will be flashed to disk by bgwriter. True... so my question is if the data block(8k) is aligned with the file system block? if it is aligned with file system block, so what's the potential issue make it is not safe for direct io. (please assume vxfs, vxvm and the disk sector is aligned ).please correct me if any incorrect. It's not about safety - it's about performance. On a machine with 64GB of RAM, a typical setting for shared_buffers set to 8GB. If you start reading blocks into the PostgreSQL cache - or writing them out of the cache - in a way that bypasses the filesystem cache, you're going to have only 8GB of cache, instead of some much larger amount. More cache = better performance. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] VX_CONCURRENT flag on vxfs( 5.1 or later) for performance for postgresql?
On Sat, Apr 30, 2011 at 4:51 AM, Hsien-Wen Chu chu.hsien@gmail.com wrote: since the block size is 8k for the default, and it consisted with many tuple/line; as my understand, if any tuple/line is changed(maybe update, insert, delete). the block will be marked as dirty block. and then it will be flashed to disk by bgwriter. True... so my question is if the data block(8k) is aligned with the file system block? if it is aligned with file system block, so what's the potential issue make it is not safe for direct io. (please assume vxfs, vxvm and the disk sector is aligned ).please correct me if any incorrect. It's not about safety - it's about performance. On a machine with 64GB of RAM, a typical setting for shared_buffers set to 8GB. If you start reading blocks into the PostgreSQL cache - or writing them out of the cache - in a way that bypasses the filesystem cache, you're going to have only 8GB of cache, instead of some much larger amount. More cache = better performance. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] VX_CONCURRENT flag on vxfs( 5.1 or later) for performance for postgresql?
Hi Mr. Greg Smith since the block size is 8k for the default, and it consisted with many tuple/line; as my understand, if any tuple/line is changed(maybe update, insert, delete). the block will be marked as dirty block. and then it will be flashed to disk by bgwriter. so my question is if the data block(8k) is aligned with the file system block? if it is aligned with file system block, so what's the potential issue make it is not safe for direct io. (please assume vxfs, vxvm and the disk sector is aligned ).please correct me if any incorrect. thank you very much Tony On 4/30/11, Greg Smith g...@2ndquadrant.com wrote: On 04/30/2011 12:24 AM, Hsien-Wen Chu wrote: I'm little bit confuse why it is not safe. and my question is following. for database application, we need to avoid double cache, PostgreSQL shared_buffer will cache the data, so we do not want to file system to cache the data right?. so the DIRECT IO is better, right?. No. There are parts of PostgreSQL that expect the operating system to do write caching. Two examples are the transaction logs and the processing done by VACUUM. If you eliminate that with direct I/O, the slowdown can be much, much larger than what you gain by eliminating double-buffering on reads. On the read side, PostgreSQL also expects that operating system features like read-ahead are working properly. While this does introduce some double-buffering, the benefits for sequential scans are larger than that overhead, too. You may not get the expected read-ahead behavior if you use direct I/O. Direct I/O is not a magic switch that makes things faster; you have to very specifically write your application to work around what it does, good and bad, before it is expected to improves things. And PostgreSQL isn't written that way. It definitely requires OS caching to work well. for VXFS, if the we use ioctl(fd,vx_cacheset,vx_concurrent) API, according to the vxfs document, it will hold a shared lock for write operation, but not the exclusive clock, also it is a direct IO, There are very specific technical requirements that you must follow when using direct I/O. You don't get direct I/O without also following its alignment needs. Read the Direct I/O best practices section of http://people.redhat.com/msnitzer/docs/io-limits.txt for a quick intro to the subject. And there's this additional set of requirements you mention in order for this particular VXFS feature to work, which I can't even comment on. But you can be sure PostgreSQL doesn't try to do either of those things--it's definitely not aligning for direct I/O. Has nothing to do with ACID or the filesystem. Now, the VXFS implementation may do some tricks that bypass the alignment requirements. But even if you got it to work, it would still be slower for anything but some read-only workloads. Double buffering is really not that big of a performance problem, you just need to make sure you don't set shared_buffers to an extremely large value. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com 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@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
[PERFORM] VX_CONCURRENT flag on vxfs( 5.1 or later) for performance for postgresql?
Dear all When database files are on a VxFS filesystem, performance can be significantly improved by setting the VX_CONCURRENT cache advisory on the file according to vxfs document, my question is that have any tested by this? #include sys/fs/vx_ioctl.h ioctl(fd, VX_SETCACHE, VX_CONCURRENT); Regards HSIEN WEN -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] VX_CONCURRENT flag on vxfs( 5.1 or later) for performance for postgresql?
On 04/27/2011 11:33 PM, HSIEN-WEN CHU wrote: When database files are on a VxFS filesystem, performance can be significantly improved by setting the VX_CONCURRENT cache advisory on the file according to vxfs document, That won't improve performance, and it's not safe either. VX_CONCURRENT switches the filesystem to use direct I/O. That's usually slower for PostgreSQL. And it introduces some requirements for both block alignment and the application avoiding overlapping writes. PostgreSQL doesn't do either, so I wouldn't expect it to be compatible with VX_CONCURRENT. -- Greg Smith 2ndQuadrant USg...@2ndquadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
