At 05/11/2017 04:11 AM, Xiaochu Liu wrote:
Hi there,
I'm trying to tune compression options for btrfs. Specifically, I want
to know the performance impact on the system under different
compression unit (block) sizes.
Compression unit size is fixed in btrfs.
It's sectorsize, determined at mkfs time, and only 4K (page size) is
supported for x86 yet.
I'm aware of '--nodesize' parameter which sets the block size of
metadata tree. Does that also set the block size in an extent? (from
my understanding, file data are mostly stored in extent unless small
enough to be inline-d in metadata leaf node?)
nodesize only affects metadata, nothing to do with data size.
Also from btrfs's wikipedia page:
In compressed extents, individual blocks are not compressed
separately; rather, the compression stream spans the entire extent.
Is that still true?
Yes.
For example if there is one continuous range represents 0~1M data of one
file, and all this data is dirty (not written to disk).
Then compress will happen when trying to writing them to disk.
And since the maximum uncompressed size for compressed extent is 128K
(fixed), the 0~1M will be split into 8 extents (if compression ratio is
acceptable).
And then each 128K extent will be compressed then compressed data will
be written to disk. (compressed extent still meet sectorsize alignment).
So the wiki page is still right and we must read out the whole
(compressed) extent to get its content.
And since both uncompressed data and compressed extent must meet
sectorsize alignment, data smaller than or equal to sectorsize won't go
through compression since it will just waste CPU time and no space saving.
Thanks,
Qu
Thanks,
Xiaochu
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