Re: [PATCH v8 03/10] md: Neither resync nor reshape while the system is frozen

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Tue, 2017-10-10 at 15:30 -0700, Shaohua Li wrote:
> On Tue, Oct 10, 2017 at 02:03:39PM -0700, Bart Van Assche wrote:
> > Some people use the md driver on laptops and use the suspend and
> > resume functionality. Since it is essential that submitting of
> > new I/O requests stops before a hibernation image is created,
> > interrupt the md resync and reshape actions if the system is
> > being frozen. Note: the resync and reshape will restart after
> > the system is resumed and a message similar to the following
> > will appear in the system log:
> > 
> > md: md0: data-check interrupted.
> 
> Where do we restart resync and reshape?

Hello Shaohua,

My understanding of the md driver is as follows:
- Before any write occurs, md_write_start() is called. That function clears
  mddev->safemode and checks mddev->in_sync. If that variable is set, it is
  cleared and md_write_start() wakes up mddev->thread and waits until the
  superblock has been written to disk.
- All mddev->thread implementations call md_check_recovery(). That function
  updates the superblock by calling md_update_sb() if needed.
  md_check_recovery() also starts a resync thread if the array is not in
  sync. See also the comment block above md_check_recovery().
- md_do_sync() performs the resync. If it completes successfully the
  "in_sync" state is set (set_in_sync()). If it is interrupted the "in_sync"
  state is not modified.
- super_90_sync() sets the MD_SB_CLEAN flag in the on-disk superblock if the
  "in_sync" flag is set.

In other words: if the array is in sync the MD_SB_CLEAN flag is set in the
on-disk superblock. If a resync is needed that flag is not set. The
MD_SB_CLEAN flag is checked when the superblock is loaded. If that flag is
not set a resync is started.

Bart.