Re: [PATCH] btrfs-progs: balance: Sync the fs before balancing metadata chunks

[David Sterba <dsterba@suse.cz>]

On Tue, Feb 26, 2019 at 01:55:06PM +0800, Qu Wenruo wrote:
> 
> 
> On 2019/2/26 上午1:21, David Sterba wrote:
> > On Tue, Jan 29, 2019 at 02:57:39PM +0800, Qu Wenruo wrote:
> >> The most straightforward is to sync the fs before balancing metadata
> >> chunks.
> >>
> >> We could enhance the kernel bytes_may_use calculation, but I doubt about
> >> the complexity.
> >> So I take the easy fix to reduce the false ENOSPC reports.
> > 
> > Agreed.
> > 
> >> +	/*
> >> +	 * There may be many over-reserved space for metadata block groups,
> >> +	 * especially for inlined file extents.
> >> +	 *
> >> +	 * Do a sync here will free those over-reserved space and hugely
> >> +	 * reduce the possibility of some false ENOSPC
> >> +	 */
> >> +	if (args->flags & BTRFS_BALANCE_METADATA) {
> >> +		ret = btrfs_util_sync(path);
> > 
> > As the fd is already open, we should use the _fd version,
> > 
> >> +		if (ret) {
> >> +			error("failed to sync the fs before balance: %m");
> >> +			ret = -errno;
> >> +			goto out;
> > 
> > and possibly only warn if there's an error returned as the sync failure
> > is not a critical condition.
> 
> AFAIK if we can't even sync the fs, the balance is definitely going to
> fail, as the most common failure mode for syncfs is RO fs, caused by
> aborted transaction.
> 
> Thus I still think we should error out. Or is there some other
> non-critical failure mode I missed?

The read-only filesystem will be checked when balance starts, that's
where it gets reported.