Messages in this thread | | | Date | Sat, 24 Nov 2001 12:36:18 -0600 | From | Steve Bergman <> | Subject | Disk hardware caching, performance, and journalling |
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Hi,
I made a couple of discoveries today which were surprising to me:
1. Disk hardware caching defaults to ON. (hdparm -W1 /dev/hda) 2. It makes a *big* difference in write performance.
I had always thought that the default was off. I also always assumed that a small cache behind a large (OS) cache would make no difference.
Here are my results with bonnie under kernel 2.4.14 on a reiserfs with a maxtor Diamond max+ 60GB udma100 drive:
Write caching on: -------Sequential Output-------- ---Sequential Input-- --Random-- -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks--- Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU 256 12618 97.1 38027 36.3 9647 6.9 11250 73.6 31832 12.1 200.9 1.2
Write caching off: -------Sequential Output-------- ---Sequential Input-- --Random-- -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks--- Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU 256 9917 76.3 12280 11.2 5159 3.5 9934 65.3 33056 14.1 203.9 1.4 Note that block writes are over 3 times faster with caching on.
So what are the implications here for journalling? Do I have to turn off caching and suffer a huge performance hit?
-Steve Bergman
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