Re: What's the best hardver for PostgreSQL 8.1?

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

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