Few missing notes to this email:
- my performance testing, results and conclusion were done only on
mechanical drives (hitachi 7k500 and wd re 500) and only with meta-data
intensive workload. Basically tar -xf src.tar; unmount and rm -rf src;
unmount where src.tar was src.tar of stable at that time.
- at the same time WAPBL was submitted to tech@ and IIRC it increased
perf a lot since I've also been using limited checksum blocks caching
(not in patch, not submitted yet) and since WAPBL log is on constant
place RAID1c/s was more happy.
- at the time I've not had any ssd/nvme for testing. Situation may be
different with this especially once someone think what's tolerable and
what's not anymore w.r.t. speed.
On 1/12/20 9:58 PM, Karel Gardas wrote:
Tried something like that in the past:
https://marc.info/?l=openbsd-tech&m=144217941801350&w=2
It worked kind of OK except the performance. The problem is that data
layout makes read op. -> 2x read op. and write op. -> read op. + 2x
write op. which is not the speed winner. Caching of checksuming blocks
helped in some cases a lot, but was not submitted since you would also
ideally need readahead and this was not done at all. The other perf
issue is that putting this slow virtual drive impl. under already slow
ffs is a receipt for disappointment from the perf. point of view.
Certainly no speed daemon and certainly completely different league
than checkumming able fss from open-source world (ZFS, btrfs,
bcachefs. No HAMMER2 is not there since it checksum only meta-data and
not user data and can't self-heal).
Yes, you are right that ideally drive would be fs aware to optimize
rebuild, but this may be worked around by more clever layout marking
also used blocks. Anyway, that's (and above) are IMHO reasons why
development is done on checksumming fss instead of checksumming
software raids. Read somewhere paper about linux's mdadm hacked to do
checksums and the result was pretty much the same (IIRC!). E.g. perf.
disappointment. If you are curious, google for it.
So, work on it if you can tolerate the speed...
