07-10-2014
Quote:
Originally Posted by
Corona688
I can see why it would cause issues when contiguous pages aren't available, but why using contiguous pages cause pages to become discontinuous?
Oracle doesn't technically request "contiguous" pages, it requests large pages. To get those large pages, the OS must coalesce smaller pages.
Oracle uses the large page, releases it, then something else requests normal size pages and the large page gets fragmented. Oracle comes back and requests larges pages...
Rinse, lather, repeat.
The coalescing necessary to create the large pages can have some nasty performance impacts as it tends to lock up virtual memory management while it's happening. And for any OS instance, VM management tends to get very single threaded when memory gets tight. Processes won't start or hang because fork() and brk() calls block while the virtual memory manager thrashes about.
This can be really bad with Oracle on Solaris using ZFS because the ZFS ARC cache uses lots of small pages and isn't exactly quick in letting them go, and the standard recommendation is "let the ARC cache get as big as it wants, it doesn't hurt anything." Umm, wrong.
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LEARN ABOUT DEBIAN
e4defrag
E4DEFRAG(8) System Manager's Manual E4DEFRAG(8)
NAME
e4defrag - online defragmenter for ext4 filesystem
SYNOPSIS
e4defrag [ -c ] [ -v ] target ...
DESCRIPTION
e4defrag reduces fragmentation of extent based file. The file targeted by e4defrag is created on ext4 filesystem made with "-O extent"
option (see mke2fs(8)). The targeted file gets more contiguous blocks and improves the file access speed.
target is a regular file, a directory, or a device that is mounted as ext4 filesystem. If target is a directory, e4defrag reduces fragmen-
tation of all files in it. If target is a device, e4defrag gets the mount point of it and reduces fragmentation of all files in this mount
point.
OPTIONS
-c Get a current fragmentation count and an ideal fragmentation count, and calculate fragmentation score based on them. By seeing this
score, we can determine whether we should execute e4defrag to target. When used with -v option, the current fragmentation count and
the ideal fragmentation count are printed for each file.
Also this option outputs the average data size in one extent. If you see it, you'll find the file has ideal extents or not. Note
that the maximum extent size is 131072KB in ext4 filesystem (if block size is 4KB).
If this option is specified, target is never defragmented.
-v Print error messages and the fragmentation count before and after defrag for each file.
NOTES
e4defrag does not support swap file, files in lost+found directory, and files allocated in indirect blocks. When target is a device or a
mount point, e4defrag doesn't defragment files in mount point of other device.
Non-privileged users can execute e4defrag to their own file, but the score is not printed if -c option is specified. Therefore, it is
desirable to be executed by root user.
AUTHOR
Written by Akira Fujita <a-fujita@rs.jp.nec.com> and Takashi Sato <t-sato@yk.jp.nec.com>.
SEE ALSO
mke2fs(8), mount(8).
e4defrag version 2.0 May 2009 E4DEFRAG(8)