Re: Flaw with SATA disks - not suitable for deduplication environment

On Thu, Jun 05, 2008 at 06:54:15PM +0800, Chong, Jenson wrote:

Hi,

Anybody have insights on this article? Does our ONTAP "Lost Write Protection" address this issue?

I don't have any particular insights, but I believe that you're reasonably safe with RAID DP and block level checksums (or is that ECC?).

Netapp formats drives with 520 bytes per sector, for the specific reason to add 8 extra bytes of checksum/ECC code per block. So ONtap knows when a block is bad, single bit flips from faulty SATA drives can't fool it. Also, the weekly scrub should highlight those faults, so they don't pile up in unused corners of your filesystem.

RAID DP will make sure that during reconstruction of a single failed drive, single bit errors on the other drives have a backup on the second parity.

However, having said that... in the about 5 years that we deploy SATA drives with netapp, we've had one case where data could possibly have been lost (so we had to re-initialize the snapmirror. It was only used as backup), and another where data was lost that was not currently in use by the filesystem. This mainly happened when we had simultaneous disk failures (3 or more drives in the same machine, max 2 in the same raid group).

We've never seen 2 simultaneous drive failures (within the rebuild time) with FC drives in the 10+ years we use those.

So, to recap: there is some truth in this article, but it's certainly not as bad as it sounds, with ONtap. And even normal drives have CRCs appended to sectors, so I cannot imagine bits just flipping without the drivers giving a warning, even on other systems.

Regards, Jenson

A bit of a flaw with SATA disk drives

June 03, 2008
By Jerome Wendt
Network World Asia

High-capacity serial ATA (SATA) disk drives are now a

mainstay in many storage systems and make it feasible for almost any company to obtain a storage system with terabytes of capacity at a reasonable cost. Yet these systems reveal a specific, known deficiency of SATA disk drives that demand companies exercise caution as to what environments they deploy these systems into.

A minor flaw with SATA disk drives that high capacity

storage systems expose is their bit error rate. Bit errors occur infrequently - about once for every 100 trillion bits. However RAID technology, which is normally used by storage systems to protect against data loss, does not detect if a specific bit on a SATA drive becomes unreadable.

While this is normally not a problem on smaller systems,

as storage systems add more capacity, the issue becomes more acute. On systems with more than 10TB of capacity the probability of a specific bit of data becoming unreadable is a distinct possibility. On systems with over 100TB, it becomes almost a certainty.

So the question becomes: Does losing access to one bit

of data really matter? Often, it doesn't unless one stores deduplicated data on these systems which is now the fastest growing trend in data storage. When data is deduplicated, the storage system's need to read every bit of data becomes paramount. The inability to access even a bit of data can result in multiple files becoming unreadable since they all may depend on a specific bit of data to complete their reconstruction.

High capacity SATA-based storage systems are the answer

to many companies' archiving and backup problems. But SATA bits can bite and using SATA drives to store large amounts of deduplicated data is not always the match made in heaven that vendors make them out to be.

Jerome Wendt is the president and lead analyst at DCIG

Inc. You may read his blogs at www.dciginc.com http://www.dciginc.com/ .