Re: Bad performance with near-full FS

[Tovo Rabemanantsoa <tovo.rabemanantsoa@bordeaux.inra.fr>]

On 06/15/2015 03:29 PM, David Sterba wrote:
> On Mon, Jun 15, 2015 at 02:47:24PM +0200, Tovo Rabemanantsoa wrote:
>> Hi all,
>> By browsing this list's archive, I've found a thread initiated by
>> Charles Cazabon entitled: "Oddly slow read performance with near-full
>> largish FS."
>> Actually, I'm living the same experience but with a not so large FS
>> (256GB on a SSD). Indeed, when I have less than 1GB of free space, the
>> applications (thunderbird, thunar ...) on the machine become awfully
>> slow but remain normal if I make some cleaning.
>> Is it due to the FS or because it's an SSD hard disk ?
> 
> 1G of 256G is less than a percent. At this level of usage you can expect
> slowdown on any filesystem.
> 
> This could be caused by free space fragmentation and even on a SSD, this
> needs extra time to process.  Higher number of fragments needs more
> structures to represent them and cost more CPU time, though this still
> might not be the worst impact.
> 
> AFAIK btrfs space handling logic needs to do more flushes of unwritten
> data when the accounted free space goes below some threshold (because
> COW needs to write the data twice before it switches to the new "root"
> pointer and can free the previous version).

Thanks for you reply,
If I really understand, it's always a good idea to keep more than 1% of
free space. Right ?