Hello everybody :) first time participant on linux-btrfs@vger.kernel.org mailinglist, hence please excuse (yet tell me about) any problems. thank you.
My question/topic is: Wanting to generate backups of a btrfs filesystems on a running system it seems that using `btrfs subvolume snapshot` would be a possible way to make certain that data kept in RAM (i.e. buffer/cache) would be synced to the disk. Reading this mailing list I stumpled upon this: >> Subject: Re: freezes during snapshot creation/deletion -- to be expected? >> (Was: Re: btrfs based backup?) >> From: Zygo Blaxell <ce3g8jdj () umail ! furryterror ! org> >> >> [..] >> >> Snapshot create has unbounded running time on 5.0 kernels. The creation >> process has to flush dirty buffers to the filesystem to get a clean >> snapshot state. Any process that is writing data while the flush is >> running gets its data included in the snapshot flush, so in the worst >> possible case, the snapshot flush never ends (unless you run out of disk >> space, or whatever was writing new data stops, whichever comes first). >> [..] Now I wonder that if `btrfs filesystem sync` would be a viable alternative to `btrfs subvolume snapshot`, with respect of not having to risk a "snapshot flush never ends" situation? My layman perception is that. 1) "btrfs on-disk-persistet data is ideally alway non-corrupted". Since changes are commited via COW and hence in a atomic fashion, meaning that at worst data on disk is outdated, but never corrupt. (unless hardware or blockdevice issues ) 2) btrfs filesystem sync or sync(1) should flush data out from memory to disk - which would once finished - lead to a "more recent" consistent data on disk. 3) btrfs subvolume snapshot implies a sync Are those perceptions roughly correct? If so I am unsure if the issue with a "neverending flush" is related to the btrfs filesystem sync and consequently relying on btrfs filesystem sync as alternative to btrfs snapshot to prevent "a neverending flush" is not a possibility. Tahnk yo and best regards, Alexander Mahr
