# mount -t ext4 /dev/hda1 /wherever
- - 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, such
- as ext3, 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.
+ - When comparing performance with other filesystems, it's always
+ important to try multiple workloads; very often a subtle change in a
+ workload parameter can completely change the ranking of which
+ filesystems do well compared to others. When comparing versus ext3,
+ note that ext4 enables write barriers by default, while ext3 does
+ not enable write barriers by default. So it is useful to use
+ explicitly specify whether barriers are enabled or not when via the
+ '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
+ for a fair comparison. When tuning ext3 for best benchmark numbers,
+ it is often worthwhile to try changing the data journaling mode; '-o
+ data=writeback,nobh' can be faster for some workloads. (Note
+ however that running mounted with data=writeback can potentially
+ leave stale data exposed in recently written files in case of an
+ unclean shutdown, which could be a security exposure in some
+ situations.) Configuring the filesystem with a large journal can
+ also be helpful for metadata-intensive workloads.
2. Features
===========
* ability to use filesystems > 16TB (e2fsprogs support not available yet)
* 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
+* internal redundancy in tree
* improved file allocation (multi-block alloc)
* fix 32000 subdirectory limit
* nsec timestamps for mtime, atime, ctime, create time
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.
-
-noextents ext4 will not use extents for newly created files
+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.
journal_checksum Enable checksumming of the journal transactions.
This will allow the recovery code in e2fsck and the
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.
+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.
data=journal All data are committed into the journal prior to being
written into the main file system.
debug Extra debugging information is sent to syslog.
-errors=remount-ro(*) Remount the filesystem read-only on an error.
+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.
+ (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.
nodelalloc Disable delayed allocation. Blocks are allocation
when data is copied from user to page cache.
+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
+ transactoin 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 priorty) 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.
+
Data Mode
=========
There are 3 different data modes:
projects / linux-2.6-omap-h63xx.git / blobdiff