Re: Speed up mount time?

[Roman Mamedov <rm@romanrm.net>]

On Wed, 23 Nov 2022 13:33:17 +0100
Joakim <ahoj79@gmail.com> wrote:

> Initially mount times were quite long, about 10 minutes. But after i
> did run a defrag with -c option on machine B the mount time increased
> to over 30 minutes. This volume has a little over 100 TB stored.
> 
> How come the mount time increased by this?

With compression each file now has to be split into extents each at most 128K
long, which means there are much more extents to keep track of than before.

> And is there any way to decrease the mount times? 10 minutes is long
> but acceptable, while 30 minutes is way too long.
> 
> Advice would be highly appreciated. :)

It is unfortunate that you use the bare sdb device, if you used LVM, you could
easily add an SSD and deploy LVM caching in writethrough mode (or a RAID1 of
SSDs and use the writeback mode). In my experience LVM caching of HDD based
storage to SSDs improves mount times of Btrfs really by a lot.

There have been a number of attempts to implement Btrfs native storage tiering
of HDDs vs SSDs (storing metadata on the latter), such as [1], but none have
made it so far, and the patches from 2020 probably no longer apply.

[1]https://lwn.net/ml/linux-btrfs/20201029053556.10619-1-wangyugui@e16-tech.com/

-- 
With respect,
Roman