标签：linux page cache vs direct io
Overview
| Block device layer |
| page cache |
| IO scheduer |
Page cache contains all file I/O data, direct I/O bypasses the page cache.
Page cache helps Linux to economize I/O
– Read requests can be made faster by adding a read ahead quantity, depending on the
historical behavior of file system accesses by applications
– Write requests are delayed and data in the page cache can have multiple updates before
being written to disk.
– Write requests in the page cache can be merged into larger I/O requests
But page cache...
– Requires Linux memory pages
– Is not useful when cached data is not exploited
Data just only needed once
Application buffers data itself
– In Linux does not know which data the application really needs next. It makes only a guess
No alternatives if application cannot handle direct I/O
Consider to use...
direct I/O:
– bypasses the page cache
– is a good choice in all cases where the application does not want Linux to economize I/O and/or where the application buffers larger amount of file contents
async I/O:
– prevents the application from being blocked in the I/O system call until the I/O completes
– allows read merging by Linux in case of using page cache
– can be combined with direct I/O
temporary files:
– should not reside on real disks, a ram disk or tmpfs allows fastest access to these files
– they don‘t need to survive a crash, don‘t place them on a journaling file system
file system:
– use ext3 and select the appropriate journaling mode (journal, ordered, writeback)
– turning off atime is only suitable if no application makes decisions on "last read" time,consider relatime instead
Direct I/O versus Page cache
Direct I/O
– Preferable if application caches itself
Application knows best which data is needed again
Application knows which data is most likely needed next
Example database base management systems DBMS
– Preferable if caching makes no sense
Data only needed once
Backup and restore
Page cache
– Optimizes re-read / write but can be critical
Data written to the page cache but not to disk yet can get lost if data loss cannot easily be handled
– If application cannot handle direct I/O
Typical example is a file server
标签：linux page cache vs direct io
原文地址：http://xmwang.blog.51cto.com/9889672/1614204
