2017-06-05 19:10 GMT+03:00 David Sterba <dsterba@xxxxxxx>:
> On Tue, May 30, 2017 at 02:18:05AM +0300, Timofey Titovets wrote:
>> Btrfs already skip store of data where compression didn't
>> free at least one byte. Let's make logic better and make check
>> that compression free at least one sector size
>> because in another case it useless to store this data compressed
>>
>> Signed-off-by: Timofey Titovets <nefelim4ag@xxxxxxxxx>
>> ---
>> fs/btrfs/inode.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index 17cbe930..2793007b 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -609,9 +609,10 @@ static noinline void compress_file_range(struct inode *inode,
>> /*
>> * one last check to make sure the compression is really a
>> * win, compare the page count read with the blocks on disk
>> + * compression must free at least one sector size
>> */
>> total_in = ALIGN(total_in, PAGE_SIZE);
>> - if (total_compressed >= total_in) {
>> + if (total_compressed + blocksize > total_in) {
>
> We're doing aligned calculation here, shouldn't this be >= ?
>
> If total_compressed + blocksize == total_in, we have saved exactly one
> blocksize.
We discussed that already in:
https://www.mail-archive.com/linux-btrfs@xxxxxxxxxxxxxxx/msg64350.html
IIRC: long in short:
- input data size 8192
- output data size 4096
This invertion logic, i.e. check if compression can be skipped, if
comparasion are true -> skip compression.
In case of above check,
old logic:
total_compressed >= total_in
4096 >= 8192 -> will_compress=1
With if blocksize added:
4096+4096 >= 8192 -> will_compress=0
So this must be changed to:
4096+4096 > 8192 -> will_compress=1
Because compression save one blocksize
Also will_compress not used after this code, so in theory this code
can be refactored to be more obvious, like that:
total_in = ALIGN(total_in, PAGE_SIZE);
- if (total_compressed + blocksize > total_in) {
- will_compress = 0;
- } else {
+ if (total_compressed + blocksize <= total_in) {
num_bytes = total_in;
*num_added += 1;
Thanks
--
Have a nice day,
Timofey.
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