XFS write cache flush policy

[Lin Li]

Hi, Guys. I recently suffered a huge data loss on power cut on an XFS
partition. The problem was that I copied a lot of files (roughly 20Gb) to
an XFS partition, then 10 hours later, I got an unexpected power cut. As a
result, all these newly copied files disappeared as if they had never been
copied. I tried to check and repair the partition, but xfs_check reports no
error at all. So I guess the problem is that the meta data for these files
were all kept in the cache (64Mb) and were never committed to the hard
disk.

What is the cache flush policy for XFS? Does it always reserve some fixed
space in cache for metadata? I asked because I thought since I copied such
a huge amount of data, at least some of these files must be fully committed
to the hard disk, then cache is only 64Mb anyway. But the reality is all of
them were lost. the only possibility I can think is some part of the cache
was reserved for meta data, so even the cache is fully filled, this part
will not be written to the disk. Am I right?
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