[lkml]   [2020]   [Aug]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: [PATCH v3] exfat: remove EXFAT_SB_DIRTY flag
    >> When should VOL_DIRTY be cleared?
    >> The current behavior is ...
    >> Case of mkdir, rmdir, rename:
    >> - set VOL_DIRTY before operation
    >> - set VOL_CLEAN after operating.
    >> In async mode, it is actually written to the media after 30 seconds.
    >> Case of cp, touch:
    >> - set VOL_DIRTY before operation
    >> - however, VOL_CLEAN is not called in this context.
    >> VOL_CLEAN will call by sync_fs or unmount.
    >> I added VOL_CLEAN in last of __exfat_write_inode() and exfat_map_cluster().
    >> As a result, VOL_DIRTY is cleared with cp and touch.
    >> However, when copying a many files ...
    >> - Async mode: VOL_DIRTY is written to the media twice every 30 seconds.
    >> - Sync mode: Of course, VOL_DIRTY and VOL_CLEAN to the media for each
    >> file.
    >> Frequent writing VOL_DIRTY and VOL_CLEAN increases the risk of boot-
    >> sector curruption.
    >> If the boot-sector corrupted, it causes the following serious problems on
    >> some OSs.
    >> - misjudge as unformatted
    >> - can't judge as exfat
    >> - can't repair
    >> I want to minimize boot sector writes, to reduce these risk.
    >> I looked vfat/udf implementation, which manages similar dirty information
    >> on linux, and found that they ware mark-dirty at mount and cleared at
    >> unmount.
    >> Here are some ways to clear VOL_DIRTY.
    >> (A) VOL_CLEAN after every write operation.
    >> :-) Ejectable at any time after a write operation.
    >> :-( Many times write to Boot-sector.
    >> (B) dirty at mount, clear at unmount (same as vfat/udf)
    >> :-) Write to boot-sector twice.
    >> :-( It remains dirty unless unmounted.
    >> :-( Write to boot-sector even if there is no write operation.
    >> (C) dirty on first write operation, clear on unmount
    >> :-) Writing to boot-sector is minimal.
    >> :-) Will not write to the boot-sector if there is no write operation.
    >> :-( It remains dirty unless unmounted.
    >> (D) dirty on first write operation, clear on sync-fs/unmount
    >> :-) Writing to boot-sector can be reduced.
    >> :-) Will not write to the boot-sector if there is no write operation.
    >> :-) sync-fs makes it clean and ejectable immidiately.
    >> :-( It remains dirty unless sync-fs or unmount.
    >> :-( Frequent sync-fs will increases writes to boot-sector.
    >> I think it should be (C) or(D).
    >> What do you think?
    > First of all, I'm sorry for the late reply.
    > And thank you for the suggestion.

    Thanks for thinking on this complicated issue.

    > Most of the NAND flash devices and HDDs have wear leveling and bad sector replacement algorithms applied.
    > So I think that the life of the boot sector will not be exhausted first.

    I'm not too worried about the life of the boot-sector.
    I'm worried about write failures caused by external factors.
    (power failure/system down/vibration/etc. during writing)
    They rarely occur on SD cards, but occur on many HDDs, some SSDs and USB storages by 0.1% or more.
    Especially with AFT-HDD, not only boot-sector but also the following multiple sectors become unreadable.
    It is not possible to completely solve this problem, as long as writing to the boot-sector.
    (I think it's a exFAT's specification defect)
    The only effective way to reduce this problem is to reduce writes to the boot-sector.

    > Currently the volume dirty/clean policy of exfat-fs is not perfect,

    Thank you for sharing the problem with you.

    > but I think it behaves similarly to the policy of MS Windows.

    On Windows10, the dirty flag is cleared after more than 15 seconds after all write operations are completed.
    (dirty-flag is never updated during the write operation continues)

    > Therefore,
    > I think code improvements should be made to reduce volume flag records while maintaining the current policy.

    Current policy is inconsistent.
    As I wrote last mail, the problem with the current implementation is that the dirty-flag may not be cleared
    after the write operation.(even if sync is enabled or disabled)
    Because, some write operations clear the dirty-flag but some don't clear.
    Unmount or sync command is the only way to ensure that the dirty-flag is cleared.
    This has no effect on clearing the dirty-flag after a write operations, it only increases risk of destroying
    the boot-sector.
    - Clear the dirty-flag after every write operation.
    - Never clear the dirty-flag after every write operation.
    Unless unified to either one, I think that sync policy cannot be consistent.

    How do you think?

    etsuhiro Kohada <>

     \ /
      Last update: 2020-08-12 11:21    [W:11.142 / U:0.000 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site