Ext4 Filesystem =============== This is a development version of the ext4 filesystem, an advanced level of the ext3 filesystem which incorporates scalability and reliability enhancements for supporting large filesystems (64 bit) in keeping with increasing disk capacities and state-of-the-art feature requirements. Mailing list: linux-ext4@vger.kernel.org 1. Quick usage instructions: =========================== - Grab updated e2fsprogs from ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs-interim/ This is a patchset on top of e2fsprogs-1.39, which can be found at ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/ - It's still mke2fs -j /dev/hda1 - mount /dev/hda1 /wherever -t ext4dev - To enable extents, mount /dev/hda1 /wherever -t ext4dev -o extents - The filesystem is compatible with the ext3 driver until you add a file which has extents (ie: `mount -o extents', then create a file). NOTE: The "extents" mount flag is temporary. It will soon go away and extents will be enabled by the "-o extents" flag to mke2fs or tune2fs - When comparing performance with other filesystems, remember that ext3/4 by default offers higher data integrity guarantees than most. So when comparing with a metadata-only journalling filesystem, use `mount -o data=writeback'. And you might as well use `mount -o nobh' too along with it. Making the journal larger than the mke2fs default often helps performance with metadata-intensive workloads. 2. Features =========== 2.1 Currently available * ability to use filesystems > 16TB * extent format reduces metadata overhead (RAM, IO for access, transactions) * extent format more robust in face of on-disk corruption due to magics, * internal redunancy in tree 2.1 Previously available, soon to be enabled by default by "mkefs.ext4": * dir_index and resize inode will be on by default * large inodes will be used by default for fast EAs, nsec timestamps, etc 2.2 Candidate features for future inclusion There are several under discussion, whether they all make it in is partly a function of how much time everyone has to work on them: * improved file allocation (multi-block alloc, delayed alloc; basically done) * fix 32000 subdirectory limit (patch exists, needs some e2fsck work) * nsec timestamps for mtime, atime, ctime, create time (patch exists, needs some e2fsck work) * inode version field on disk (NFSv4, Lustre; prototype exists) * reduced mke2fs/e2fsck time via uninitialized groups (prototype exists) * journal checksumming for robustness, performance (prototype exists) * persistent file preallocation (e.g for streaming media, databases) Features like metadata checksumming have been discussed and planned for a bit but no patches exist yet so I'm not sure they're in the near-term roadmap. The big performance win will come with mballoc and delalloc. CFS has been using mballoc for a few years already with Lustre, and IBM + Bull did a lot of benchmarking on it. The reason it isn't in the first set of patches is partly a manageability issue, and partly because it doesn't directly affect the on-disk format (outside of much better allocation) so it isn't critical to get into the first round of changes. I believe Alex is working on a new set of patches right now. 3. Options ========== When mounting an ext4 filesystem, the following option are accepted: (*) == default extents ext4 will use extents to address file data. The file system will no longer be mountable by ext3. journal=update Update the ext4 file system's journal to the current format. journal=inum When a journal already exists, this option is ignored. Otherwise, it specifies the number of the inode which will represent the ext4 file system's journal file. journal_dev=devnum When the external journal device's major/minor numbers have changed, this option allows the user to specify the new journal location. The journal device is identified through its new major/minor numbers encoded in devnum. noload Don't load the journal on mounting. data=journal All data are committed into the journal prior to being written into the main file system. data=ordered (*) All data are forced directly out to the main file system prior to its metadata being committed to the journal. data=writeback Data ordering is not preserved, data may be written into the main file system after its metadata has been committed to the journal. commit=nrsec (*) Ext4 can be told to sync all its data and metadata every 'nrsec' seconds. The default value is 5 seconds. This means that if you lose your power, you will lose as much as the latest 5 seconds of work (your filesystem will not be damaged though, thanks to the journaling). This default value (or any low value) will hurt performance, but it's good for data-safety. Setting it to 0 will have the same effect as leaving it at the default (5 seconds). Setting it to very large values will improve performance. barrier=1 This enables/disables barriers. barrier=0 disables it, barrier=1 enables it. orlov (*) This enables the new Orlov block allocator. It is enabled by default. oldalloc This disables the Orlov block allocator and enables the old block allocator. Orlov should have better performance - we'd like to get some feedback if it's the contrary for you. user_xattr Enables Extended User Attributes. Additionally, you need to have extended attribute support enabled in the kernel configuration (CONFIG_EXT4_FS_XATTR). See the attr(5) manual page and http://acl.bestbits.at/ to learn more about extended attributes. nouser_xattr Disables Extended User Attributes. acl Enables POSIX Access Control Lists support. Additionally, you need to have ACL support enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL). See the acl(5) manual page and http://acl.bestbits.at/ for more information. noacl This option disables POSIX Access Control List support. reservation noreservation bsddf (*) Make 'df' act like BSD. minixdf Make 'df' act like Minix. check=none Don't do extra checking of bitmaps on mount. nocheck debug Extra debugging information is sent to syslog. errors=remount-ro(*) Remount the filesystem read-only on an error. errors=continue Keep going on a filesystem error. errors=panic Panic and halt the machine if an error occurs. grpid Give objects the same group ID as their creator. bsdgroups nogrpid (*) New objects have the group ID of their creator. sysvgroups resgid=n The group ID which may use the reserved blocks. resuid=n The user ID which may use the reserved blocks. sb=n Use alternate superblock at this location. quota noquota grpquota usrquota bh (*) ext4 associates buffer heads to data pages to nobh (a) cache disk block mapping information (b) link pages into transaction to provide ordering guarantees. "bh" option forces use of buffer heads. "nobh" option tries to avoid associating buffer heads (supported only for "writeback" mode). Data Mode --------- There are 3 different data modes: * writeback mode In data=writeback mode, ext4 does not journal data at all. This mode provides a similar level of journaling as that of XFS, JFS, and ReiserFS in its default mode - metadata journaling. A crash+recovery can cause incorrect data to appear in files which were written shortly before the crash. This mode will typically provide the best ext4 performance. * ordered mode In data=ordered mode, ext4 only officially journals metadata, but it logically groups metadata and data blocks into a single unit called a transaction. When it's time to write the new metadata out to disk, the associated data blocks are written first. In general, this mode performs slightly slower than writeback but significantly faster than journal mode. * journal mode data=journal mode provides full data and metadata journaling. All new data is written to the journal first, and then to its final location. In the event of a crash, the journal can be replayed, bringing both data and metadata into a consistent state. This mode is the slowest except when data needs to be read from and written to disk at the same time where it outperforms all others modes. References ========== kernel source: <file:fs/ext4/> <file:fs/jbd2/> programs: http://e2fsprogs.sourceforge.net/ http://ext2resize.sourceforge.net useful links: http://fedoraproject.org/wiki/ext3-devel http://www.bullopensource.org/ext4/