Hi, William,
William Yu wrote:
> Random write performance (small block that only writes to 1 drive):
> 1 write requires N-1 reads + N writes --> 1/2N-1 %
This is not true. Most Raid-5 engines use XOR or similar checksum
methods. As opposed to cryptographic checksums, those can be updated and
corrected incrementally.
check_new = check_old xor data_old xor data_new
So 2 reads and 2 writes are enough: read data and checksum, then adjust
the checksum via the data difference, and write data and new checksum.
And often, the old data block still is in cache, accounting to 1 read
and two writes.
HTH,
Markus
--
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf. | Software Development GIS
Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org
