Re: [PATCH 0/1] nilfs2: add mount option that reduces super block writes

[Andreas Rohner <andreas.rohner-hi6Y0CQ0nG0@public.gmane.org>]

Hi Ryusuke,

On 2014-01-30 06:29, Ryusuke Konishi wrote:
> Hi Andreas,
> On Thu, 30 Jan 2014 03:46:59 +0100, Andreas Rohner wrote:
>> Hi,
>>
>> This is only a hacky proof of concept implementation and probably 
>> full of nasty bugs. I also havn't really tested it. I was 
>> just interested how hard it would be to implement Clemens' suggestion 
>> of writing the super block only at umount time and do a linear scan 
>> of all the segments in case of file system failure.
>> The linear scan is only performed if the file system wasn't shut down 
>> properly. So for normal operation there shouldn't be any slowdown.
> 
> This premise is not acceptable.
> We have to avoid long mount time even after unexpected power failures.
> 
> I prefer some sort of way which combines binary search of segment
> summary blocks and partial linear scan of logs. 
> 
> I don't know the latency of recent SSDs, however we should estimate
> the latency of disk I/O about 5ms~20ms per a separate block (in the
> case of hard drives).  So the maximum number of scans of segment
> summary blocks seems to be roughly 10~100 times.
> 
> Regards,
> Ryusuke Konishi

Basically I agree with you. It was just a quick experiment. I just
thought Clemens suggestion, to have a mount option to turn on the linear
scan for users who want it, was worth a try.

br,
Andreas Rohner

>>
>> I repurposed the ss_pad field of nilfs_segment_summary to contain the 
>> crc seed, because I needed a way to distinguish left over segments 
>> from previous nilfs2 volumes from real segments that are part of the 
>> current file system. 
>>
>> I don't really expect it to be merged or anything. Maybe it can spark 
>> a discussion. Maybe somebody could try it out on an old SD-Card and 
>> time the mount command or something.
>>
>> I tested it on a virtual machine. It seemed to recover fine when I 
>> killed the VM and mounted it again. Clearly more testing is 
>> necessary...
>>
>> br,
>> Andreas Rohner
>>
>> ---
>> Andreas Rohner (1):
>>   nilfs2: add mount option that reduces super block writes
>>
>>  fs/nilfs2/segbuf.c        |  3 ++-
>>  fs/nilfs2/segment.c       |  3 ++-
>>  fs/nilfs2/super.c         | 10 +++++++--
>>  fs/nilfs2/the_nilfs.c     | 54 ++++++++++++++++++++++++++++++++++++++++++++---
>>  include/linux/nilfs2_fs.h |  4 +++-
>>  5 files changed, 66 insertions(+), 8 deletions(-)
>>
>> -- 
>> 1.8.5.3
>>
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