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From: Dominik Csapak <d.csapak@proxmox.com>
To: pbs-devel@lists.proxmox.com
Date: Fri, 4 Nov 2022 10:49:34 +0100
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information. [datastore.rs]
Subject: [pbs-devel] [PATCH proxmox-backup v5] datastore: make 'filesystem'
the default sync-level
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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>
---
changes from v4:
* included benchmark & rationale in the commit message
docs/storage.rst | 4 ++--
pbs-api-types/src/datastore.rs | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/docs/storage.rst b/docs/storage.rst
index c4e44c72..d61c3a40 100644
--- a/docs/storage.rst
+++ b/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
diff --git a/pbs-api-types/src/datastore.rs b/pbs-api-types/src/datastore.rs
index 4c9eda2f..ff8fe55f 100644
--- a/pbs-api-types/src/datastore.rs
+++ b/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,
}
--
2.30.2