[linux-lvm] is lvmraid 1 thinpool with integrity safe?

[Gionatan Danti]

Il 2023-04-09 14:21 Arvid Picciani ha scritto:
> Hi,
> 
> doing some performance tests i noticed that lvmraid + integrity +
> thinpool outperforms zfs z1 by 5x while offering the same features.
> (snapshots, integrity)
> 
> Is this somehow unsafe or how come it is so unpopular?
> 
> lvcreate --type raid1 --mirrors 1 --size '100M' loop --name loopmeta
> --raidintegritymode journal
> lvcreate --type raid1 --mirrors 1 --size '700M' loop --name loopdata
> --raidintegritymode journal
> lvconvert --type thin-pool --poolmetadata loop/loopmeta loop/loop

I benchmarked this setup quite heavily some years ago (when RHEL 8 was 
shiny new), using integritysetup because lvm integration was not 
available.

Be aware that in journal mode all random writes are sequentially 
accumulated in the journal and are later destaged. If using HDDs this 
last part can block your array for extended time. For example, using 
fio, I noticed 50k random writes for 2s followed by 30s where random 
iops was 0 (zero). Bitmap mode (rather than journal) was better 
speed-wise, but be sure to understand the tradeoff.

Other notes regarding ZFS:
- ZRAID1 is the equivalent of RAID5 and has lower performance than 
mirrors;
- recordsize plays an important role, but can affect benchmark results 
in a non-intuitive manner;
- ZFS is heavily dependent on ARC - don't try to outsmart it with 
syntethical tests, rather benchmark it your real workload.

Regards.

-- 
Danti Gionatan
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