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Make data preallocation tunable

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Make mount options for the size of preallocation and whether or not it
should be restricted to extending writes.  Disabling the default
restriction to streaming writes lets it preallocate in aligned regions
of the preallocation size when they contain no extents.

Signed-off-by: Zach Brown <zab@versity.com>
This commit is contained in:
Zach Brown
2022-10-05 15:19:56 -07:00
parent 064409eb62
commit ef2daf8857
5 changed files with 246 additions and 51 deletions
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34
utils/man/scoutfs.5
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@@ -21,6 +21,40 @@ as detailed in
.BR acl (5) .
Support for POSIX ACLs is the default.
.TP
.B data_prealloc_blocks=<blocks>
Set the size of preallocation regions of data files, in 4KiB blocks.
Writes to these regions that contain no extents will attempt to
preallocate the size of the full region. This can waste a lot of space
with small files, files with sparse regions, and files whose final
length isn't a multiple of the preallocation size. The following
data_prealloc_contig_only option, which is the default, restricts this
behaviour to waste less space.
.sp
All the preallocation options can be changed in an active mount by
writing to their respective files in the options directory in the
mount's sysfs directory.
.sp
It is worth noting that it is always more efficient in every way to use
.BR fallocate (2)
to precisely allocate large extents for the resulting size of the file.
Always attempt to enable it in software that supports it.
.TP
.B data_prealloc_contig_only=<0|1>
This option, currently the default, limits file data preallocation in
two ways. First, it will only preallocate when extending a fully
allocated file. Second, it will limit the size of preallocation to the
existing length of the file. These limits reduce the amount of
preallocation wasted per file at the cost of multiple initial extents in
all files. It only supports simple streaming writes, any other write
pattern will not be recognized and could result in many fragmented
extent allocations.
.sp
This option can be disabled to encourage large allocated extents
regardless of write patterns. This can be helpful if files are written
with initial sparse regions (perhaps by multiple threads writing to
different regions) and wasted space isn't an issue (perhaps because the
file population contains few small files).
.TP
.B metadev_path=<device>
The metadev_path option specifies the path to the block device that
contains the filesystem's metadata.