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 <li...@tigertech.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/ > > >
