Am Sonntag, 17. Februar 2013 schrieb Subranshu Patel: > I created 2 filesystem on my system (RHEL 6.3, kernel version 2.6.32) > - XFS and EXT4 and mounted them. > > On both the filesystem I executed the mdtest tool(opensource tool) for > 64 concurrent process. Each process performed the following: > - Create large number of directories > - Remove all the directories > > During this time I monitored the memory usage of the system using sar > command. I checked the 3 components - kbmemused, kbbuffers and > kbcached > > kbmemused - Amount of used memory in kilobytes. This does not take > into account memory used by the kernel itself. > kbbuffers - buffer cache > kbcached - page cache > > While the kbmemused and kbcached component was almost similar in EXT4 > and XFS (XFS being a little higher), the kbbuffer showed a totally > different trend. > > For EXT4, kbbuffers was: > 390999KB for dir creation > 364803KB for dir removal > For XFS, kbbuffers was: > > 1701KB for dir creation > 2738KB for dir removal > > In kernel 2.6, both buffer cache and page cache are merged. The page > cache caches pages of files. The buffer cache caches disk blocks which > consists of mainly metadata (not file data). > > Why is the buffer cache large in case of EXT4 and what is stored in > the buffer cache? What is stored in the buffer cache? An interesting question. I also wondered about it. I always thought filesystem metadata that is to be written to the disk. As opposed to dirty pages which are counted in Dirty: in /proc/meminfo. Then on being asked in a Linux Performance Analyse and Tuning training I held where I had some little Linux kernel hackers in there, it seemed to me, they found out, that it is a disk block buffer by looking at the source. And indeed on doing dd if=/dev/zero of=/dev/somedevice bs=1M or so the buffer count raises considerably. What I never really understand was what is the clear distinction between dirty pages and disk block buffers. Why isn´t anything that is about to be written to disk in one cache? Can anybody enlighten me? PS: buffers=0 with BTRFS also. Thanks, -- Martin 'Helios' Steigerwald - http://www.Lichtvoll.de GPG: 03B0 0D6C 0040 0710 4AFA B82F 991B EAAC A599 84C7 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
