On 2017-09-03 19:55, Qu Wenruo wrote:
On 2017年09月04日 02:06, Adam Borowski wrote:
On Sun, Sep 03, 2017 at 07:32:01PM +0200, Cloud Admin wrote:
Hi,
I used the mount option 'compression' on some mounted sub volumes. How
can I revoke the compression? Means to delete the option and get all
data uncompressed on this volume.
Is it enough to remount the sub volume without this option? Or is it
necessary to do some addional step (balancing?) to get all stored data
uncompressed.
If you set it via mount option, removing the option is enough to disable
compression for _new_ files. Other ways are chattr +c and
btrfs-property,
but if you haven't heard about those you almost surely don't have such
attributes set.
After remounting, you may uncompress existing files. Balancing won't do
this as it moves extents around without looking inside; defrag on the
other
hand rewrites extents thus as a side effect it applies new
[non]compression
settings. Thus: ｢btrfs fi defrag -r /path/to/filesystem｣.
Beside of it, is it possible to find out what the real and compressed
size
of a file, for example or the ratio?
Currently not.
I've once written a tool which does this, but 1. it's extremely slow, 2.
insane, 3. so insane a certain member of this list would kill me had I
distributed the tool. Thus, I'd need to rewrite it first...
AFAIK the only method to determine the compression ratio is to check the
EXTENT_DATA key and its corresponding file_extent_item structure.
(Which I assume Adam is doing this way)
In that structure is records its on-disk data size and in-memory data
size. (All rounded up to sectorsize, which is 4K in most case)
So in theory it's possible to determine the compression ratio.
The only method I can think of (maybe I forgot some methods?) is to use
offline tool (btrfs-debug-tree) to check that.
FS APIs like fiemap doesn't even support to report on-disk data size so
we can't use it.
But the problem is more complicated, especially when compressed CoW is
involved.
For example, there is an extent (A) which represents the data for inode
258, range [0,128k).
On disk size its just 4K.
And when we write the range [32K, 64K), which get CoWed and compressed,
resulting a new file extent (B) for inode 258, range [32K, 64K), and on
disk size is 4K as an example.
Then file extent layout for 258 will be:
[0,32k): range [0,32K) of uncompressed Extent A
[32k, 64k): range [0,32k) of uncompressed Extent B
[64k, 128k): range [64k, 128K) of uncompressed Extent A.
And on disk extent size is 4K (compressed Extent A) + 4K (compressed
Extent B) = 8K.
Before the write, the compresstion ratio is 4K/128K = 3.125%
While after write, the compression ratio is 8K/128K = 6.25%
Not to mention that it's possible to have uncompressed file extent.
So it's complicated even we're just using offline tool to determine the
compression ratio of btrfs compressed file.
Out of curiosity, is there any easier method if you just want an
aggregate ratio for the whole filesystem? The intuitive option of
comparing `du -sh` output to how much space is actually used in chunks
is obviously out because that will count sparse ranges as 'compressed',
and there should actually be a significant difference in values there
for an uncompressed filesystem (the chunk usage should be higher).
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