Re: synchronization of XFS

[Steve Lord <lord@xxxxxxx>]

Stefan Smietanowski wrote:
> 
> Yup. That's what's happening. It first does one run with --just-copy
> where it writes the files using the filesystem then reads the same
> files using the blockdevice and it's own filesystem code basically.
> 
> // Stefan
> 

And I presume the files are missing?

The bizzare part of this is that if you read the via the block
device interface, you are looking at the same in memory pages
which xfs uses for the metadata cache. So even if the data has
not hit disk yet, things such as names in directories should
be visible in the metadata cache. Inodes may be more tricky,
since flushing of inodes into the metadata cache is delayed.

Try this for an experiment, before the run, set

/proc/sys/fs/xfs/sync_interval

down to some small number like 1000, pause for a couple
of seconds after calling sync, and then see if grub
can see the files via the block device.

This tunable controls how long xfs delays writing out
inodes into the metadata cache from their internal format.

Steve