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

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

On 2014-01-30 08:44, Vyacheslav Dubeyko wrote:
> 
> On Jan 30, 2014, at 9:02 AM, Andreas Rohner wrote:
> 
>> Hi Vyacheslav,
>>
>> On 2014-01-30 07:36, Vyacheslav Dubeyko wrote:
>>> Hi Andreas,
>>>
>>> On Jan 30, 2014, at 5:47 AM, Andreas Rohner wrote:
>>>
>>>> This patch introduces a mount option bad_ftl that disables the
>>>> periodic overwrites of the super block to make the file system better
>>>> suitable for bad flash memory with a bad FTL. The super block is only
>>>> written at umount time. So if there is a power outage the file system
>>>> needs to be recovered by a linear scan of all segment summary blocks.
>>>>
>>>> The linear scan is only necessary if the file system wasn't umounted
>>>> properly. So the normal mount time is not affected.
>>>>
>>>> Signed-off-by: Andreas Rohner <andreas.rohner-hi6Y0CQ0nG0@public.gmane.org>
>>>> ---
>>>> 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(-)
>>>>
>>>
>>> As far as I can judge, conceptually it is simply rollback of the fix [1].
>>
>> The normal recovery mode checks all partial segments and computes the
>> checksum over all the data. That takes significantly longer than my
>> approach of just checking one block per segment, namely the segment
>> summary block.
>>
> 
> I don't think that your suggestion changes situation significantly.
> Because in the issue [1] you can go through the whole volume in the really
> bad environment. Maybe you will mount volume 30 minutes instead of 1 hour
> with technique that you suggest in the patch.
>>> [1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=a9bae189542e71f91e61a4428adf6e5a7dfe8063

I have just finished a test with my  100 GB HDD and SSD. I filled it
with dd until it was 100% full. Then I cut power to the machine and
timed the following mount operation:

100GB HDD:
time sudo mount -o bad_ftl /dev/sda1 /mnt/

real	1m21.068s
user	0m0.020s
sys	0m0.770s

100GB SSD:
time sudo mount -o bad_ftl /dev/sdc1 /mnt/

real	0m2.124s
user	0m0.010s
sys	0m0.243s

So it looks quite bad for hard drives. To scan a 1 TB hard drive would
take 13 minutes.

But a 1 TB SSD would only take 20 seconds!

I will test one of my SD cards next.

Regards,
Andreas Rohner
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