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Documentation: zram: correct algo parameters configuration documentation

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zram has always reset all previously set parameters for the given
algorithm in comp_params_store().  Make documentation more clear and
explicitly state that all relevant/necessary parameters should be set in
one configuration write.

Link: https://lore.kernel.org/20260730075158.1339787-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Sergey Senozhatsky
2026-07-30 16:51:46 +09:00
committed by Andrew Morton
parent 9477820c63
commit f74e6dff54
1 changed files with 36 additions and 4 deletions
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40
Documentation/admin-guide/blockdev/zram.rst
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@@ -109,14 +109,41 @@ path to the `dict` along with other parameters::
#pass path to pre-trained zstd dictionary
echo "algo=zstd dict=/etc/dictionary" > /sys/block/zram0/algorithm_params
#same, but using algorithm priority
echo "priority=1 dict=/etc/dictionary" > \
/sys/block/zram0/algorithm_params
#pass path to pre-trained zstd dictionary and compression level
echo "algo=zstd level=8 dict=/etc/dictionary" > \
/sys/block/zram0/algorithm_params
#same, but using algorithm priority
echo "algo=zstd priority=1" > /sys/block/zram0/recomp_algorithm
echo "priority=1 dict=/etc/dictionary" > \
/sys/block/zram0/algorithm_params
Each write to `algorithm_params` replaces the entire set of parameters of
the corresponding algorithm, parameters that are not listed in the write
are reset to their default values. Configure all of the parameters of an
algorithm in one write::
#WRONG: the second write resets level back to its default value
echo "algo=zstd level=8" > /sys/block/zram0/algorithm_params
echo "algo=zstd dict=/etc/dictionary" > /sys/block/zram0/algorithm_params
#RIGHT
echo "algo=zstd level=8 dict=/etc/dictionary" > \
/sys/block/zram0/algorithm_params
Select the compression algorithm before configuring its parameters. The
parameters of one algorithm are not necessarily valid for another one, so
changing the algorithm of a particular priority resets that priority's
parameters::
#WRONG: comp_algorithm write resets the previously configured level
echo "level=8" > /sys/block/zram0/algorithm_params
echo zstd > /sys/block/zram0/comp_algorithm
#RIGHT
echo zstd > /sys/block/zram0/comp_algorithm
echo "algo=zstd level=8" > /sys/block/zram0/algorithm_params
Parameters are algorithm specific: not all algorithms support pre-trained
dictionaries, not all algorithms support `level`. Furthermore, for certain
algorithms `level` controls the compression level (the higher the value the
@@ -124,6 +151,11 @@ better the compression ratio, it even can take negatives values for some
algorithms), for other algorithms `level` is acceleration level (the higher
the value the lower the compression ratio).
Parameters are handed over to the compression algorithm when the device is
initialised, hence invalid parameters (or parameters that the selected
algorithm does not support) are reported by the `disksize` write, and not
by the `algorithm_params` write that has configured them.
Set Disksize
============