>> If you are completely over-writing an entire stripe, there's no reason to
>> read the existing data; you would just calculate the parity information from
>> the new data. Any good controller should take that approach.
>
> in theory yes, in practice the OS writes usually aren't that large and aligned,
> and as a result most raid controllers (and software) don't have the
> special-case code to deal with it.
I'm pretty sure all half-decent controllers and software do actually. This is
one major reason that large (hopefully battery backed) caches help RAID-5
disproportionately. The larger the cache the more likely it'll be able to wait
until the entire raid stripe is replaced avoid having to read in the old
parity.
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's 24x7 Postgres support!
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