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If the SSD is no faster, it means the bottleneck is elsewhere, and the second hour of downtime won't happen. There will instead be a brief second outage while I put the old disc back, then teaparty will run slow for another 24 hours while it resyncs the old second HDD.
So the outage happened, and the SSD was temporarily installed. Interestingly, tests revealed it was no faster than the old HDD, and in some cases slower. So the discs themselves aren't the bottleneck.
I took the opportunity of being at the colo to test another hypothesis, which was that UEFI systems are now much more common than they used to be, and the new kernel is basically designed to work with systems that booted via UEFI (which isn't how teaparty is installed) rather than old-fashioned BIOS (which it is).
This seems to be the root of the problem. The current secondary disc, sdb, writes at about 10-15MB/s when booted under BIOS, but 150MB/s when booted UEFI.
There is a process for turning a BIOS-booting system into a UEFI-booting one, but it's quite fiddly, and (naturally) involves more downtime. At the moment, I expect this will happen this Wednesday, 20/3, and will probably last most of the morning, and possibly most of the day.
Again, if this will be a disaster for you, please let me know asap.
- Tom