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. It is only activated by the mount option bad_ftl. So the user has to chose it explicitly. The user wouldn't activate it if he/she uses a 4 TB hard drive. The user also wouldn't activate it if he/she uses a modern SSD with a decent FTL. It would only make sense to activate it for crappy SD-Cards. > I prefer some sort of way which combines binary search of segment > summary blocks and partial linear scan of logs. For a binary search the segments have to be sorted at some granularity (groups). I think this would hinder more sophisticated GC policies. That seems to be a high price just so that the super block is not updated so often. > 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 > >> >> 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 >> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-nilfs" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-nilfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
