Re: Slow startup of systemd-journal on BTRFS

[Martin Steigerwald <Martin@lichtvoll.de>]

Am Samstag, 14. Juni 2014, 02:53:20 schrieb Duncan:
> > I am reaching the conclusion that fallocate is not the problem. The
> > fallocate increase the filesize of about 8MB, which is enough for some
> > logging. So it is not called very often.
> 
> But... 
> 
> If a file isn't (properly[1]) set NOCOW (and the btrfs isn't mounted with 
> nodatacow), then an fallocate of 8 MiB will increase the file size by 8 
> MiB and write that out.  So far so good as at that point the 8 MiB should 
> be a single extent.  But then, data gets written into 4 KiB blocks of 
> that 8 MiB one at a time, and because btrfs is COW, the new data in the 
> block must be written to a new location.
> 
> Which effectively means that by the time the 8 MiB is filled, each 4 KiB 
> block has been rewritten to a new location and is now an extent unto 
> itself.  So now that 8 MiB is composed of 2048 new extents, each one a 
> single 4 KiB block in size.

I always thought that the whole point of fallocate is that it *doesn´t* write 
out anything, but just reserves the space. Thus I don´t see how COW can have 
any adverse effect here.

-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
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