Re: Raid over 48 disks

[Thiemo Nagel <thiemo.nagel@ph.tum.de>]

>>         16k read        64k write
>> chunk
>> size    RAID 5  RAID 6  RAID 5  RAID 6
>> 128k    492     497     268     270
>> 256k    615     530     288     270
>> 512k    625     607     230     174
>> 1024k   650     620     170     75
> 
> It strikes me that these numbers are meaningless without knowing if
> that is actual data-to-disk or data-to-memcache-and-some-to-disk-too.
> Later versions of 'dd' offer 'conv=fdatasync' which is really handy
> (call fdatasync on the output file, syncing JUST the one file, right
> before close). Otherwise, oflags=direct will (try) to bypass the
> page/block cache.
> 
> I can get really impressive numbers, too (over 200MB/s on a single
> disk capable of 70MB/s) when I (mis)use dd without fdatasync, et al.
> 
> The variation in reported performance can be really huge without
> understanding that you aren't actually testing the DISK I/O but *some*
> disk I/O and *some* memory caching.

I did these benchmarks with 32GB of data on a machine with 1GB of RAM, 
therefore the memory cache contribution should be small.

Kind regards,

Thiemo