Re: Btrfs autodefrag wrote 5TB in one day to a 0.5TB SSD without a measurable benefit

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

On Tue, Mar 15, 2022 at 03:22:43PM -0400, Remi Gauvin wrote:
> On 2022-03-15 2:51 p.m., Zygo Blaxell wrote:
> 
> > The main advantage of larger extents is smaller metadata, and it doesn't
> > matter very much whether it's SSD or HDD.  Adjacent extents will be in
> > the same metadata page, so not much is lost with 256K extents even on HDD,
> > as long as they are physically allocated adjacent to each other.
> > 
> 
> When I tried enabling compress-force on my HDD storage, it *killed*
> sequential read performance.  I could write a file out at over
> 100MB/s... but trying to read that same file sequentially would trash
> the drives with less than 5MB/s actually being able to be read.
> 
> 
> No such problems were observed on ssd storage.

I've seen a similar effect.  I wonder if the small extents are breaking
readahead or something.

> I was under the impression this problem was caused trying to read files
> with the 127k extents,, which, for whatever reason, could not be done
> without excessive seeking.