> -----Original Message-----
> From: linux-btrfs-ow...@vger.kernel.org <linux-btrfs-
> ow...@vger.kernel.org> On Behalf Of Marc MERLIN
> Sent: Tuesday, 3 July 2018 2:16 PM
> To: Qu Wenruo <quwenruo.bt...@gmx.com>
> Cc: Su Yue <suy.f...@cn.fujitsu.com>; linux-btrfs@vger.kernel.org
> Subject: Re: how to best segment a big block device in resizeable btrfs
> filesystems?
>
> On Tue, Jul 03, 2018 at 09:37:47AM +0800, Qu Wenruo wrote:
> > > If I do this, I would have
> > > software raid 5 < dmcrypt < bcache < lvm < btrfs That's a lot of
> > > layers, and that's also starting to make me nervous :)
> >
> > If you could keep the number of snapshots to minimal (less than 10)
> > for each btrfs (and the number of send source is less than 5), one big
> > btrfs may work in that case.
>
> Well, we kind of discussed this already. If btrfs falls over if you reach
> 100 snapshots or so, and it sure seems to in my case, I won't be much better
> off.
> Having btrfs check --repair fail because 32GB of RAM is not enough, and it's
> unable to use swap, is a big deal in my case. You also confirmed that btrfs
> check lowmem does not scale to filesystems like mine, so this translates into
> "if regular btrfs check repair can't fit in 32GB, I am completely out of luck
> if
> anything happens to the filesystem"
Just out of curiosity I had a look at my backup filesystem.
vm-server /media/backup # btrfs fi us /media/backup/
Overall:
Device size: 5.46TiB
Device allocated: 3.42TiB
Device unallocated: 2.04TiB
Device missing: 0.00B
Used: 1.80TiB
Free (estimated): 1.83TiB (min: 1.83TiB)
Data ratio: 2.00
Metadata ratio: 2.00
Global reserve: 512.00MiB (used: 0.00B)
Data,RAID1: Size:1.69TiB, Used:906.26GiB
/dev/mapper/a-backup--a 1.69TiB
/dev/mapper/b-backup--b 1.69TiB
Metadata,RAID1: Size:19.00GiB, Used:16.90GiB
/dev/mapper/a-backup--a 19.00GiB
/dev/mapper/b-backup--b 19.00GiB
System,RAID1: Size:64.00MiB, Used:336.00KiB
/dev/mapper/a-backup--a 64.00MiB
/dev/mapper/b-backup--b 64.00MiB
Unallocated:
/dev/mapper/a-backup--a 1.02TiB
/dev/mapper/b-backup--b 1.02TiB
compress=zstd,space_cache=v2
202 snapshots, heavily de-duplicated
551G / 361,000 files in latest snapshot
Btrfs check normal mode took 12 mins and 11.5G ram
Lowmem mode I stopped after 4 hours, max memory usage was around 3.9G
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