[systemd-devel] [ANNOUNCE] systemd 219

[Goffredo Baroncelli]

Hi Lennart,

On 2015-02-16 23:59, Lennart Poettering wrote:
>         * journald now sets the special FS_NOCOW file flag for its
>           journal files. This should improve performance on btrfs, by
>           avoiding heavy fragmentation when journald's write-pattern
>           is used on COW file systems. It degrades btrfs' data
>           integrity guarantees for the files to the same levels as for
>           ext3/ext4 however. This should be OK though as journald does
>           its own data integrity checks and all its objects are
>           checksummed on disk. Also, journald should handle btrfs disk
>           full events a lot more gracefully now, by processing SIGBUS
>           errors, and not relying on fallocate() anymore.

If I read correctly the code, the FS_NOCOW is a temporary workaround, i.e.
when the file is closed (or rotated ?) the FS_NOCOW flags is unset again. 
It is true ?

If so, the time window where a file is un-protect by the checksum is
quite small. I was worried not about the corruption detection but about loosing 
the ability to recover the file from a good copy (if available) in case of corruption. 
But this seems limited only when the file is in use (before the next rotation).

BR
G.Baroncelli
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