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. > 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. > 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. 喵! -- ⢀⣴⠾⠻⢶⣦⠀ ⣾⠁⢰⠒⠀⣿⡁ Vat kind uf sufficiently advanced technology iz dis!? ⢿⡄⠘⠷⠚⠋⠀ -- Genghis Ht'rok'din ⠈⠳⣄⠀⠀⠀⠀ -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
