下面这些东西来自于kernel 里面的document，蓝色是我的看法 When mounting an ext4 filesystem, the following option are accepted: (*) == default
ro - Mount filesystem read only. Note that ext4 will replay the journal (and thus write to the partition) even when mounted "read only". The mount options "ro,noload" can be used to prevent writes to the filesystem. 即使ro，也会写journal,“ro,noload" 看来会禁止写到文件系统中
journal_checksum - Enable checksumming of the journal transactions,This will allow the recovery code in e2fsck and the kernel to detect corruption in the kernel. It is a compatible change and will be ignored by older kernels. journal transaction checksum
journal_async_commit - Commit block can be written to disk without waiting for descriptor blocks. If enabled older kernels cannot mount the device. This will enable 'journal_checksum' internally. 异步journal commit
journal_path=path
journal_dev=devnum - When the external journal device's major/minor numbers have changed, these options allow the user to specify the new journal location. The journal device is identified through either its new major/minor numbers encoded in devnum, or via a path to the device. 应该是控制使用external journal的
norecovery -
noload - Don't load the journal on mounting. Note that if the filesystem was not unmounted cleanly, skipping the journal replay will lead to the filesystem containing inconsistencies that can lead to any number of problems. 挂载文件系统不载入journal, 所以 “ro,noload" 文件系统只读，又不挂载journal.
data=journal - All data are committed into the journal prior to being written into the main file system. Enabling this mode will disable delayed allocation and O_DIRECT support. journal 不commit ,不会写入文件系统，enable 这个选项会disable delayed_allocation 和O_DIRECT(这个好像是直接I/O，对数据库比较好)
data=ordered (*) - All data are forced directly out to the main file system prior to its metadata being committed to the journal. 元数据没有在journal上commit,就开始写入文件系统，默认
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. 不考虑数据和元数据在journal 上commit的次序，这个选项能提升I/O性能
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. 默认5秒，设置为0和5是一样的，大了能提高性能但不见得安全，小则数据更安全但性能会受影响。
barrier=<0|1(*)> / barrier(*) / nobarrier - This enables/disables the use of write barriers in the jbd code. barrier=0 disables, barrier=1 enables. This also requires an IO stack which can support barriers, and if jbd gets an error on a barrier write, it will disable again with a warning.Write barriers enforce proper on-disk ordering of journal commits, making volatile disk write caches safe to use, at some performance penalty. If your disks are battery-backed in one way or another disabling barriers may safely improve performance. The mount options "barrier" and "nobarrier" can also be used to enable or disable barriers, for consistency with other ext4 mount options. 在有电池的块设备上nobarrier 能提高性能，没有电池的块设备应该使用barrier 来保证数据安全
inode_readahead_blks=n - This tuning parameter controls the maximum number of inode table blocks that ext4's inode table readahead algorithm will pre-read into the buffer cache. The default value is 32 blocks. 预读inode ? 32*blocksize (4k) = 128k ?
nouser_xattr - Disables Extended User Attributes. See the attr(5) manual page and more information about extended attributes
noacl - This option disables POSIX Access Control List support. If ACL support is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is enabled by default on mount. See the acl(5) manual page and for more information about acl.
bsddf (*) - Make 'df' act like BSD
minixdf - Make 'df' act like Minix.
debug - Extra debugging information is sent to syslog.
abort - Simulate the effects of calling ext4_abort() for debugging purposes. This is normally used while remounting a filesystem which is already mounted.
errors=remount-ro - Remount the filesystem read-only on an error. 最常见的，我的ubuntu /etc/fstab 根目录就是这么挂载的
errors=continue - Keep going on a filesystem error.
errors=panic - Panic and halt the machine if an error occurs. (These mount options override the errors behavior specified in the superblock, which can be configured using tune2fs)
data_err=ignore(*) - Just print an error message if an error occurs in a file data buffer in ordered mode.
data_err=abort - Abort the journal if an error occurs in a file data buffer in ordered mode.
grpid / bsdgroups -- Give objects the same group ID as their creator.
nogrpid (*) / sysvgroups -- New objects have the group ID of their creator.
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 - These options are ignored by the filesystem. They are used only by quota tools to recognize volumes where quota should be turned on. See documentation in the quota-tools package for more details (). 都是供做磁盘quota 使用的option
jqfmt= /usrjquota= /grpjquota= -- These options tell filesystem details about quota so that quota information can be properly updated during journal replay. They replace the above quota options. See documentation in the quota-tools package for more details ().
还是供做磁盘quota 使用的option
stripe=n - Number of filesystem blocks that mballoc will try to use for allocation size and alignment. For RAID5/6 systems this should be the number of data disks * RAID chunk size in file system blocks.
delalloc (*) - Defer block allocation until just before ext4 writes out the block(s) in question. This allows ext4 to better allocation decisions more efficiently.
nodelalloc - Disable delayed allocation. Blocks are allocated when the data is copied from userspace to the page cache, either via the write(2) system call or when an mmap'ed page which was previously unallocated is written for the first time.
max_batch_time=usec - Maximum amount of time ext4 should wait for additional filesystem operations to be batch together with a synchronous write operation. Since a synchronous write operation is going to force a commit and then a wait for the I/O complete, it doesn't cost much, and can be a huge throughput win, we wait for a small amount of time to see if any other transactions can piggyback on the synchronous write. The algorithm used is designed to automatically tune for the speed of the disk, by measuring the amount of time (on average) that it takes to finish committing a transaction. Call this time the "commit time". If the time that the transaction has been running is less than the commit time, ext4 will try sleeping for the commit time to see if other operations will join the transaction. The commit time is capped by the max_batch_time, which defaults to 15000us (15ms). This optimization can be turned off entirely by setting max_batch_time to 0.
min_batch_time=usec - This parameter sets the commit time (as described above) to be at least min_batch_time.It defaults to zero microseconds. Increasing this parameter may improve the throughput of multi-threaded, synchronous workloads on very fast disks, at the cost of increasing latency.
journal_ioprio=prio - The I/O priority (from 0 to 7, where 0 is the highest priority) which should be used for I/O operations submitted by kjournald2 during a commit operation. This defaults to 3, which is a slightly higher priority than the default I/O priority. kjournald2 提交的I/O的优先级，0最高，7最低，默认3，比默认I/O 稍高
auto_da_alloc(*) / noauto_da_alloc - Many broken applications don't use fsync() when replacing existing files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/ rename("foo.new", "foo"), or worse yet, fd = open("foo", O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4 will detect the replace-via-rename and replace-via-truncate patterns and force that any delayed allocation blocks are allocated such that at the next journal commit, in the default data=ordered mode, the data blocks of the new file are forced to disk before the rename() operation is committed. This provides roughly the same level of guarantees as ext3, and avoids the "zero-length" problem that can happen when a system crashes before the delayed allocation blocks are forced to disk.
noinit_itable - Do not initialize any uninitialized inode table blocks in the background. This feature may be used by installation CD's so that the install process can complete as quickly as possible; the inode table initialization process would then be deferred until the next time the file system is unmounted.
init_itable=n - The lazy itable init code will wait n times the number of milliseconds it took to zero out the previous block group's inode table. This minimizes the impact on the system performance while file system's inode table is being initialized.
discard / nodiscard(*) - Controls whether ext4 should issue discard/TRIM commands to the underlying block device when blocks are freed. This is useful for SSD devices and sparse/thinly-provisioned LUNs, but it is off by default until sufficient testing has been done. 看来以后SSD的磁盘应该enable discard了
nouid32 - Disables 32-bit UIDs and GIDs. This is for interoperability with older kernels which only store and expect 16-bit values.
block_validity / noblock_validity - This options allows to enables/disables the in-kernel facility for tracking filesystem metadata blocks within internal data structures. This allows multi-block allocator and other routines to quickly locate extents which might overlap with filesystem metadata blocks. This option is intended for debugging purposes and since it negatively affects the performance, it is off by default. debug需要，对性能没好处，default off.
dioread_lock/ dioread_nolock - Controls whether or not ext4 should use the DIO read locking. If the dioread_nolock option is specified ext4 will allocate uninitialized extent before bufferwrite and convert the extent to initialized after IO completes. This approach allows ext4 code to avoid using inode mutex, which improves scalability on high speed storages. However this does not work with data journaling and dioread_nolock option will be ignored with kernel warning. Note that dioread_nolock code path is only used for extent-based files.Because of the restrictions this options comprises it is off by default (e.g. dioread_lock).
max_dir_size_kb=n - This limits the size of directories so that any attempt to expand them beyond the specified limit in kilobytes will cause an ENOSPC error. This is useful in memory constrained environments, where a very large directory can cause severe performance problems or even provoke the Out Of Memory killer. (For example, if there is only 512mb memory available, a 176mb directory may seriously cramp the system's style.) 不晓得什么意思
i_version - Enable 64-bit inode version support. This option is off by default.
