datastore: make 'filesystem' the default sync-level

rationale is that it makes the backup much safer than 'none', but does not
incur a big of a performance hit as 'file'.

here some benchmark:

data to be backed up:
~14GiB semi-random test images between 12kiB and 4GiB
that results in ~11GiB chunks (more than ram available on the target)

PBS setup:
virtualized (on an idle machine), PBS itself was also idle
8 cores (kvm64 on Intel 12700k) and 8 GiB memory

all virtual disks are on LVM with discard and iothread on
the HDD is a 4TB Seagate ST4000DM000 drive, and the NVME is a 2TB
Crucial CT2000P5PSSD8

i tested each disk with ext4/xfs/zfs (default created with the gui)
with 5 runs each, inbetween the caches are flushed and the filesystem synced
i removed the biggest and smallest result and from the remaining 3
results built the average (percentage is relative to the 'none' result)

result:

test         none     filesystem         file
hdd - ext4   125.67s  140.39s (+11.71%)  358.10s (+184.95%)
hdd - xfs    92.18s   102.64s (+11.35%)  351.58s (+281.41%)
hdd - zfs    94.82s   104.00s (+9.68%)   309.13s (+226.02%)
nvme - ext4  60.44s   60.26s (-0.30%)    60.47s (+0.05%)
nvme - xfs   60.11s   60.47s (+0.60%)    60.49s (+0.63%)
nvme - zfs   60.83s   60.85s (+0.03%)    60.80s (-0.05%)

So all in all, it does not seem to make a difference for nvme drives,
for hdds 'filesystem' increases backup time by ~10%, while
for 'file' it largely depends on the filesystem, but always
in the range of factor ~3 - ~4

Note that this does not take into account parallel actions, such as gc,
verify or other backups.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

docs/storage.rst

@@ -344,13 +344,13 @@ and only available on the CLI:
   the crash resistance of backups in case of a powerloss or hard shutoff.
   There are currently three levels:
 
-  - `none` (default): Does not do any syncing when writing chunks. This is fast
+  - `none` : Does not do any syncing when writing chunks. This is fast
     and normally OK, since the kernel eventually flushes writes onto the disk.
     The kernel sysctls `dirty_expire_centisecs` and `dirty_writeback_centisecs`
     are used to tune that behaviour, while the default is to flush old data
     after ~30s.
 
-  - `filesystem` : This triggers a ``syncfs(2)`` after a backup, but before
+  - `filesystem` (default): This triggers a ``syncfs(2)`` after a backup, but before
     the task returns `OK`. This way it is ensured that the written backups
     are on disk. This is a good balance between speed and consistency.
     Note that the underlying storage device still needs to protect itself against

pbs-api-types/src/datastore.rs

@@ -181,7 +181,6 @@ pub enum DatastoreFSyncLevel {
     /// which reduces IO pressure.
     /// But it may cause losing data on powerloss or system crash without any uninterruptible power
     /// supply.
-    #[default]
     None,
     /// Triggers a fsync after writing any chunk on the datastore. While this can slow down
     /// backups significantly, depending on the underlying file system and storage used, it
@@ -196,6 +195,7 @@ pub enum DatastoreFSyncLevel {
     /// Depending on the setup, it might have a negative impact on unrelated write operations
     /// of the underlying filesystem, but it is generally a good compromise between performance
     /// and consitency.
+    #[default]
     Filesystem,
 }