[systemd-devel] [PATCH 10/11] Avoid doubling the efforts for /var/log/journal

[Zbigniew Jędrzejewski-Szmek]

On Wed, Jun 18, 2014 at 08:10:03PM +0200, Goffredo Baroncelli wrote:
> Hi Fink
> 
> On 06/13/2014 04:41 PM, Werner Fink wrote:
> > That is: set NOATIME, NOCOW, and NOCOMP for the journal directory
> 
> If I read correctly, you want set UN-conditionally the NOCOW behavior. Please, please, please DON'T DO that.
> 
> The NOCOW behavior is not without disadvantage: yes it increase the performance but
> the file also lost the btrfs checksum protection; when BTRFS manage the disks in RAID mode and a corruption happens, it uses the checksum to select the correct mirror during the reading. If you set UN-conditionally the NOCOW behavior you lost this capability even if the user _want it_ (and if they spend moneys in two or more disks, it is likely they _want it_).
> 
> Moreover the NOCOW flags has some "strange" behavior when a NOCOW file is snapshotted (it lost the NOCOW property); this may lead to irregular performance.
> 
> If you want it, it must be configurable at least with a sane default (which IMHO should be "do nothing", following the "least surprise" rule).
> 
> If you are looking to something like that, I suggest also to defrag the journal file before the open (but still as configurable option, and considering the "least surprise" rule).

Yes, this is too much of a hack. Various defragging approaches
seem better.

Zbyszek