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mke2fs: document bigalloc and cluster-size

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Bigalloc feature has been used for a long time, but the documentation
in mke2fs is still missing.  So add it.

Addresses-Debian-Bug: #669730

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Zheng Liu 2013-01-13 17:08:15 +08:00 committed by Theodore Ts'o
parent dd4f565d5a
commit 0f680ee8f2
1 changed files with 26 additions and 0 deletions
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26
misc/mke2fs.8.in
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@ -187,6 +187,13 @@ Check the device for bad blocks before creating the file system. If
this option is specified twice, then a slower read-write
test is used instead of a fast read-only test.
.TP
.B \-C " cluster-size"
Specify the size of cluster in bytes for filesystems using the bigalloc
feature. Valid cluster-size values are from 2048 to 256M bytes per
cluster. By default (if bigalloc is enabled and no cluster size is
otherwise specified using this option), the cluster size will be 16
times the block size.
.TP
.B \-D
Use direct I/O when writing to the disk. This avoids mke2fs dirtying a
lot of buffer cache memory, which may impact other applications running
@ -309,6 +316,10 @@ RAID parameter as part of the
option rather than manipulating the number of blocks per group.)
This option is generally used by developers who
are developing test cases.
.IP
If the bigalloc feature is enabled, the
.B \-g
option will specify the number of clusters in a block group.
.TP
.BI \-G " number-of-groups"
Specify the number of block groups that will be packed together to
@ -516,6 +527,21 @@ prefix the feature name with a caret ('^') character. The
pseudo-filesystem feature "none" will clear all filesystem features.
.RS 1.2i
.TP
.B bigalloc
This feature enables clustered allocation, so that the unit of
allocation is a power of two number of blocks. That is, each bit in the
what had traditionally been known as the block allocation bitmap now
indicates whether a cluster is in use or not, where a cluster is by
default composed of 16 blocks. This feature can decrease the time
spent on doing block allocation and brings smaller fragmentation, especially
for large files. The size can be specified using the
.B \-C option.
.IP
.B Warning:
The bigalloc feature is still under development, and may not be fully
supported with your kernel or may have various bugs. Please see the web
page http://ext4.wiki.kernel.org/index.php/Bigalloc for details.
.TP
.B dir_index
Use hashed b-trees to speed up lookups in large directories.
.TP