| |
| 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/ |
