-----Original Message----- > From: linux-btrfs-ow...@vger.kernel.org <linux-btrfs- > ow...@vger.kernel.org> On Behalf Of Nikolay Borisov > Sent: Thursday, 20 December 2018 9:44 PM > To: Qu Wenruo <quwenruo.bt...@gmx.com>; devz...@web.de; linux- > bt...@vger.kernel.org > Subject: Re: compress-force not really forcing compression? > > > > On 20.12.18 г. 2:57 ч., Qu Wenruo wrote: > > > > > > On 2018/12/19 下午11:41, devz...@web.de wrote: > >> does compress-force really force compression? > > > > It should. > > > > The only exception is block size. > > > > If the file is smaller than the sector size (4K for x86_64), then no > > compression no matter whatever the mount options are. > > What FORCE_COMPRESS does is it ensures that compression is always tried > for a file (check code in compress_file_range in the if (pages) branch). > However, if btrfs_compress_pages detects compression makes no > difference then 'if (pages)' branch is executed since will_compress will not > be set and only thing it will be do is not set BTRFS_INODE_NOCOMPRESS. > > What this all means is that with FORCE_COMPRESS future writes will also be > tried to be compressed. For example if you do some non-compressible > writes on a file with no FORCE_COMPRESS then > BTRFS_INODE_NOCOMPRESS will be set. This means that all future > invocation of inode_need_compress for this inode will return false. So if at a > later time the io pattern changes to one which is compressible then it wont' > be compressed. > > OTOH with force-compress you will also be compressing those portions of > the file which are compressible. > > IMHO the more pertinent question is : > > If a file has portions which are not easily compressible does that imply all > future writes are also incompressible. IMO no, so I think what will be prudent > is remove FORCE_COMPRESS altogether and make the code act as if it's > always on. > > Any opinions?
That is a good idea. If I turn on compression I would expect everything to be compressed, except in cases where there is no size benefit.
