Re: ZFS filesystem - supported ?

In my opinion, ext4 will solve any and all problems without a very deep
understanding of file system architecture. In short, i would stick with
ext4 unless you have a good reason not to. Maybe there is one. I have done
this a long time and never thought twice about which file system should
support my servers.

On Mon, Oct 25, 2021, 6:01 PM Robert L Mathews <lists(at)tigertech(dot)com> wrote:

> On 10/25/21 1:40 PM, Mladen Gogala wrote:
> > This is probably not the place
> > to discuss the inner workings of snapshots, but it is worth knowing that
> > snapshots drastically increase the IO rate on the file system - for
> > every snapshot. That's where the slowness comes from.
>
> I have recent anecdotal experience of this. I experiment with using
> Btrfs for a 32 TB backup system that has five 8 TB spinning disks.
> There's an average of 8 MBps of writes scattered around the disks, which
> isn't super high, obviously.
>
> The results were vaguely acceptable until I created a snapshot of it, at
> which point it became completely unusable. Even having one snapshot
> present caused hundreds of btrfs-related kernel threads to thrash in the
> "D" state almost constantly, and it never stopped doing that even when
> left for many hours.
>
> I then experimented with adding a bcache layer on top of Btrfs to see if
> it would help. I added a 2 TB SSD using bcache, partitioned as 1900 GB
> read cache and 100 GB write cache. It made very little difference and
> was still unusable as soon as a snapshot was taken.
>
> I did play with the various btrfs and bcache tuning knobs quite a bit
> and couldn't improve it.
>
> Since that test was a failure, I then decided to try the same setup with
> OpenZFS on a lark, with the same set of disks in a "raidz" array, with
> the 2 TB SSD as an l2arc read cache (no write cache). It easily handles
> the same load, even with 72 hourly snapshots present, with the default
> settings. I'm actually quite impressed with it.
>
> I'm sure that the RAID, snapshots and copy-on-write reduce the maximum
> performance considerably, compared to ext4. But on the other hand, it
> did provide the performance I expected to be possible given the setup.
> Btrfs *definitely* didn't; I was surprised at how badly it performed.
>
> --
> Robert L Mathews, Tiger Technologies, http://www.tigertech.net/
>
>
>