On Mon, Jun 28, 2021 at 1:33 AM Gionatan Danti <g.danti@xxxxxxxxxx> wrote: > > Il 2021-06-28 05:28 Stuart D Gathman ha scritto: > > Yes. I like the checksums in metadata feature for enhanced integrity > > checking. > > Until recently btrfs has issue when a LVM snapshot was mounted. It is > now solved? Pretty sure it's fixed since 4.14. https://lkml.org/lkml/2019/2/10/23 > That said, for rewrite-heavy workload (virtual machines, databases, etc) > btrfs is very slow (and disabling CoW is not a solution for me, as it > also disables checksum, compression, etc). It's not inherently slow, it's a tracking cost problem as very large numbers of extents accumulate. And it also depends on the write pattern of the guest file system. If you use Btrfs in a guest on a host using Btrfs, it's a lot more competitive. There's certainly room for improvement, possibly with some hinting to avoid writing out a metric ton of 4KiB blocks as other file systems are prone to doing, where btrfs can turn these into largely sequential writes, they lose any locality optimization the guest file system expects for subsequent reads. A lot of the locality issue is a factor on rotational devices. When talking about hundreds of thousands of extents per VM file, this has a noticeable impact on even SSDs, but the much reduced latency makes it tolerable for some scenarios. But I've seen similar problems with VM's on LVM thinp when making many snapshots and incurring cow, however temporary (like a btrfs nodatacow file that's subject to snapshots or reflink copies; or a backing file on xfs likewise reflink copied). There really isn't much better we can do than LVM thick in this regard. And if that's the standard bearer, it's not much different if you fallocate a nodatacow file. Some databases are cow friendly, notably rocksdb. And sqlite with wal enabled is at least not cow unfriendly. The worst offender seems to be postgresql but I haven't seen any benchmarking since the multiple kernel series of fsync work done on btrfs to improve the performance of databases in general; that was kernel 5.8 through 5.11. -- Chris Murphy _______________________________________________ linux-lvm mailing list linux-lvm@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
