ide write barrier support
A message that is not in the archive (sent off-list or not yet received): [email protected]
Quoted 2 lines · Jens Axboe
Completely disabling write back caching on IDE drives totally kills performance typically, they are just not meant to be driven this way.
It has a significant performance impact, but I would not call it "totally kills performance". In Section 7 of
http://www.complang.tuwien.ac.at/papers/czezatke%26ertl00/
you can find some numbers for ext2 with and without write-back caching (and for other file systems without write-back caching). The slowdown was about a factor 1.5 for an un-tar benchmark and 1 for an rm benchmark (and of course 1 for the read-dominated tar and find benchmarks).
IMO the OS should turn off write-back caching on all devices that have journaling, log-structured or soft-updated file systems mounted, unless the OS ensures correctness through write barriers, explicit flushes or somesuch. I certainly turn off write-back caching on all drives with ext3 file systems.
So I hope to see write barrier support in the Linux kernel at some point in the future.
Concerning disk behaviour, I have certainly seen disks (with write-back caching turned on) that don't write a block for many seconds, while they write other, later (from the view of the CPU) blocks. See
http://www.complang.tuwien.ac.at/anton/hdtest/
- anton
Signature · 3 lines
M. Anton Ertl Some things have to be seen to be believed [email protected] Most things have to be believed to be seen http://www.complang.tuwien.ac.at/anton/home.html