On 30-Sep-08, at 6:31 PM, Tim wrote:
On Tue, Sep 30, 2008 at 5:19 PM, Erik Trimble
<[EMAIL PROTECTED]> wrote:
To make Will's argument more succinct (<wink>), with a NetApp,
undetectable (by the NetApp) errors can be introduced at the HBA
and transport layer (FC Switch, slightly damage cable) levels.
ZFS will detect such errors, and fix them (if properly configured).
NetApp has no such ability.
Also, I'm not sure that a NetApp (or EMC) has the ability to find
bit-rot. ...
NetApp's block-appended checksum approach appears similar but is in
fact much stronger. Like many arrays, NetApp formats its drives
with 520-byte sectors. It then groups them into 8-sector blocks: 4K
of data (the WAFL filesystem blocksize) and 64 bytes of checksum.
When WAFL reads a block it compares the checksum to the data just
like an array would, but there's a key difference: it does this
comparison after the data has made it through the I/O path, so it
validates that the block made the journey from platter to memory
without damage in transit.
This is not end to end protection; they are merely saying the data
arrived in the storage subsystem's memory verifiably intact. The data
still has a long way to go before it reaches the application.
--Toby
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