On 2/17/13 12:28 AM, Andreas Dilger wrote: > On 2013-02-16, at 21:04, Subranshu Patel <spatel.ml@xxxxxxxxx> wrote: > >> 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? > > XFS does not use buffer cache, while ext[234] does use buffer cache. > > This is just a different code design. Ext4 uses the buffer cache to track metadata for journaling. Use slabtop or similar to see what xfs slab caches grow during the test; look at xfs_buf for example. -Eric -- 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
