[Gllug] A SATA Raid card worth having?

[Christian Smith]

On Thu, 2 Feb 2006, Martin A. Brooks wrote:

>Nix wrote:
>
>>Of course if you're doing small piecemeal writes then there'll be a lot
>>of extra reads to compute parity across all stripes... but small
>>piecemeal writes are a fairly uncommon thing.
>>


No extra reads necassary. XOR is commutative, so you just XOR the old
block against the parity block, then XOR the new block against the parity
block and write them both.


>
>Apart from mail servers, database servers, web caching servers, log
>servers etc etc, yes, they're fairly uncommon :)


Against the cost of actually writing the data to disk, is the time to do
an XOR across a few 1K blocks ever going to be significant? Small writers
like the above applications are almost always seek bound. A seek bound
disk will handle perhaps 200-300 transactions per second? At 1K per
transaction, that's less than 1/2 MBps per disk.

Christian


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