Re: [RFC PATCH] Btrfs: do not flush csum items of unchanged file data during treelog

[Li Zefan <lizf@cn.fujitsu.com>]

Chris Mason wrote:
> Excerpts from liubo's message of 2011-04-21 03:58:21 -0400:
>>
>> The current code relogs the entire inode every time during fsync log,
>> and it is much better suited to small files rather than large ones.
>>
>> During my performance test, the fsync performace of large files sucks,
>> and we can ascribe this to the tremendous amount of csum infos of the
>> large ones, cause we have to flush all of these csum infos into log trees
>> even when there are only _one_ change in the whole file data.  Apparently,
>> to optimize fsync, we need to create a filter to skip the unnecessary csum
>> ones, that is, the corresponding file data remains unchanged before this fsync.
>>
>> Here I have some test results to show, I use sysbench to do "random write + fsync".
>>
>> Sysbench args:
>>   - Number of threads: 1
>>   - Extra file open flags: 0
>>   - 2 files, 4Gb each
>>   - Block size 4Kb
>>   - Number of random requests for random IO: 10000
>>   - Read/Write ratio for combined random IO test: 1.50
>>   - Periodic FSYNC enabled, calling fsync() each 100 requests.
>>   - Calling fsync() at the end of test, Enabled.
>>   - Using synchronous I/O mode
>>   - Doing random write test
>>
>> Sysbench results:
>> ===
>>    Operations performed:  0 Read, 10000 Write, 200 Other = 10200 Total
>>    Read 0b  Written 39.062Mb  Total transferred 39.062Mb
>> ===
>> a) without patch:  (*SPEED* : 451.01Kb/sec)
>>    112.75 Requests/sec executed
>>
>> b) with patch:     (*SPEED* : 5.1537Mb/sec)
>>    1319.34 Requests/sec executed
> 
> Really nice results! Especially considering the small size of the patch.
> 
> But, I'd really like to look at using sub transaction ids for this, and
> then logging just the part of the inode that had changed since the last
> log commit.  It's more complex, but will also help reduce tree searches
> for the file items.
> 

And this patch forgot to mention it has compatability issue.