I am doing some comparisons between Linux and NETBSD with respect to RAID5
performance. My initial results were quite shocking in that Linux was many times
faster in not only RAID5 but also a single disk partition.
I did some further digging and found that NETBSD mounts all filesystems with 'sync' as
the default. Linux (at least RH installation) builds the fstab with 'defaults' as the
mount option. 'defaults' includes 'async' I/O as part of its options. So I wasn't
comparing apples to apples.
I can't help but wonder how most of the benchmarks that have been posted to this list
were run - probably via the default of 'async'.
Under Linux too, the difference between a sync and async mounted fs is tremendous.
The below quick tests show just that, both on the same filesystem mounted differently.
Mounted as 'async'
-------Sequential Output-------- ---Sequential Input-- --Random--
-Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
100 6669 89.4 37285 31.7 18604 41.6 7126 94.4 187111 100.5 9240.9 99.3
Mounted as 'sync'
-------Sequential Output-------- ---Sequential Input-- --Random--
-Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
Machine MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
100 2769 38.7 4496 5.7 20597 45.9 7356 97.1 187016 98.6 9238.7 99.3
When publishing benchmarks please include as part of your description. When running
servers with raid devices, I would expect sync to be the safer option in case of a
crash.
Doug Egan
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