On Mar 16, 2010, at 4:41 PM, Tonmaus wrote: >> Are you sure that you didn't also enable >> something which >> does consume lots of CPU such as enabling some sort >> of compression, >> sha256 checksums, or deduplication? > > None of them is active on that pool or in any existing file system. Maybe the > issue is particular to RAIDZ2, which is comparably recent. On that occasion: > does anybody know if ZFS reads all parities during a scrub?
Yes > Wouldn't it be sufficient for stale corruption detection to read only one > parity set unless an error occurs there? No, because the parity itself is not verified. >> The main concern that one should have is I/O >> bandwidth rather than CPU >> consumption since "software" based RAID must handle >> the work using the >> system's CPU rather than expecting it to be done by >> some other CPU. >> There are more I/Os and (in the case of mirroring) >> more data >> transferred. > > What I am trying to say is that CPU may become the bottleneck for I/O in case > of parity-secured stripe sets. Mirrors and simple stripe sets have almost 0 > impact on CPU. So far at least my observations. Moreover, x86 processors not > optimized for that kind of work as much as i.e. an Areca controller with a > dedicated XOR chip is, in its targeted field. All x86 processors you care about do XOR at memory bandwidth speed. XOR is one of the simplest instructions to implement on a microprocessor. The need for a dedicated XOR chip for older "hardware RAID" systems is because they use very slow processors with low memory bandwidth. Cheap is as cheap does :-) However, the issue for raidz2 and above (including RAID-6) is that the second parity is a more computationally complex Reed-Solomon code, not a simple XOR. So there is more computing required and that would be reflected in the CPU usage. -- richard ZFS storage and performance consulting at http://www.RichardElling.com ZFS training on deduplication, NexentaStor, and NAS performance Atlanta, March 16-18, 2010 http://nexenta-atlanta.eventbrite.com Las Vegas, April 29-30, 2010 http://nexenta-vegas.eventbrite.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
