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diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt new file mode 100644 index 0000000..6a4adca --- /dev/null +++ b/Documentation/filesystems/ext4.txt @@ -0,0 +1,236 @@ + +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/ |
