On 2017年09月04日 08:14, Adam Borowski wrote:
On Mon, Sep 04, 2017 at 07:55:27AM +0800, Qu Wenruo wrote:
On 2017年09月04日 02:06, Adam Borowski wrote:
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.
BTRFS_IOC_TREE_SEARCH_V2 returns all we want to know; its only downside is
being root only.
Just forgot that.
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%
There's no real meaningful way to speak about compression ratio of a partial
extent. Thus, I decided to, for every extent, take compressed:uncompressed
sizes of the whole extent, no matter whether the file uses only a few bytes
of that extent or references it a thousand times.
Very clever move.
Not to mention that it's possible to have uncompressed file extent.
Yeah, the tool gives a report like:
all 74% 9.2M/ 13M
lzo 68% 7.1M/ 11M
none 100% 2.1M/ 2.1M
as you typically have a mix of compressible and uncompressible data.
Looks quite nice!
Thanks,
Qu
喵!
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